diff --git a/src/static/js/ace.js b/src/static/js/ace.js
index 0d273b85b..fd6fca379 100644
--- a/src/static/js/ace.js
+++ b/src/static/js/ace.js
@@ -28,19 +28,88 @@ const hooks = require('./pluginfw/hooks');
 const pluginUtils = require('./pluginfw/shared');
 
 const debugLog = (...args) => {};
-window.debugLog = debugLog;
 
 // The inner and outer iframe's locations are about:blank, so relative URLs are relative to that.
 // Firefox and Chrome seem to do what the developer intends if given a relative URL, but Safari
 // errors out unless given an absolute URL for a JavaScript-created element.
 const absUrl = (url) => new URL(url, window.location.href).href;
 
-const scriptTag =
-    (source) => `<script type="text/javascript">\n${source.replace(/<\//g, '<\\/')}</script>`;
+const eventFired = async (obj, event, cleanups = [], predicate = () => true) => {
+  if (typeof cleanups === 'function') {
+    predicate = cleanups;
+    cleanups = [];
+  }
+  await new Promise((resolve, reject) => {
+    let cleanup;
+    const successCb = () => {
+      if (!predicate()) return;
+      debugLog(`Ace2Editor.init() ${event} event on`, obj);
+      cleanup();
+      resolve();
+    };
+    const errorCb = () => {
+      const err = new Error(`Ace2Editor.init() error event while waiting for ${event} event`);
+      debugLog(`${err} on object`, obj);
+      cleanup();
+      reject(err);
+    };
+    cleanup = () => {
+      cleanup = () => {};
+      obj.removeEventListener(event, successCb);
+      obj.removeEventListener('error', errorCb);
+    };
+    cleanups.push(cleanup);
+    obj.addEventListener(event, successCb);
+    obj.addEventListener('error', errorCb);
+  });
+};
+
+const pollCondition = async (predicate, cleanups, pollPeriod, timeout) => {
+  let done = false;
+  cleanups.push(() => { done = true; });
+  // Pause a tick to give the predicate a chance to become true before adding latency.
+  await new Promise((resolve) => setTimeout(resolve, 0));
+  const start = Date.now();
+  while (!done && !predicate()) {
+    if (Date.now() - start > timeout) throw new Error('timeout');
+    await new Promise((resolve) => setTimeout(resolve, pollPeriod));
+    debugLog('Ace2Editor.init() polling');
+  }
+  if (!done) debugLog('Ace2Editor.init() poll condition became true');
+};
+
+// Resolves when the frame's document is ready to be mutated:
+//   - Firefox seems to replace the frame's contentWindow.document object with a different object
+//     after the frame is created so we need to wait for the window's load event before continuing.
+//   - Chrome doesn't need any waiting (not even next tick), but on Windows it never seems to fire
+//     any events. Eventually the document's readyState becomes 'complete' (even though it never
+//     fires a readystatechange event), so this function waits for that to happen to avoid returning
+//     too soon on Firefox.
+//   - Safari behaves like Chrome.
+// I'm not sure how other browsers behave, so this function throws the kitchen sink at the problem.
+// Maybe one day we'll find a concise general solution.
+const frameReady = async (frame) => {
+  // Can't do `const doc = frame.contentDocument;` because Firefox seems to asynchronously replace
+  // the document object after the frame is first created for some reason. ¯\_(ツ)_/¯
+  const doc = () => frame.contentDocument;
+  const cleanups = [];
+  try {
+    await Promise.race([
+      eventFired(frame, 'load', cleanups),
+      eventFired(frame.contentWindow, 'load', cleanups),
+      eventFired(doc(), 'load', cleanups),
+      eventFired(doc(), 'DOMContentLoaded', cleanups),
+      eventFired(doc(), 'readystatechange', cleanups, () => doc.readyState === 'complete'),
+      // If all else fails, poll.
+      pollCondition(() => doc().readyState === 'complete', cleanups, 10, 5000),
+    ]);
+  } finally {
+    for (const cleanup of cleanups) cleanup();
+  }
+};
 
 const Ace2Editor = function () {
   let info = {editor: this};
-  window.ace2EditorInfo = info; // Make it accessible to iframes.
   let loaded = false;
 
   let actionsPendingInit = [];
@@ -109,16 +178,19 @@ const Ace2Editor = function () {
   // returns array of {error: <browser Error object>, time: +new Date()}
   this.getUnhandledErrors = () => loaded ? info.ace_getUnhandledErrors() : [];
 
-  const pushStyleTagsFor = (buffer, files) => {
+  const addStyleTagsFor = (doc, files) => {
     for (const file of files) {
-      buffer.push(`<link rel="stylesheet" type="text/css" href="${absUrl(encodeURI(file))}"/>`);
+      const link = doc.createElement('link');
+      link.rel = 'stylesheet';
+      link.type = 'text/css';
+      link.href = absUrl(encodeURI(file));
+      doc.head.appendChild(link);
     }
   };
 
   this.destroy = pendingInit(() => {
     info.ace_dispose();
     info.frame.parentNode.removeChild(info.frame);
-    delete window.ace2EditorInfo;
     info = null; // prevent IE 6 closure memory leaks
   });
 
@@ -135,109 +207,127 @@ const Ace2Editor = function () {
       `../static/skins/${clientVars.skinName}/pad.css?v=${clientVars.randomVersionString}`,
     ];
 
-    const doctype = '<!doctype html>';
+    const skinVariants = clientVars.skinVariants.split(' ').filter((x) => x !== '');
 
-    const iframeHTML = [];
-
-    iframeHTML.push(doctype);
-    iframeHTML.push(`<html class='inner-editor ${clientVars.skinVariants}'><head>`);
-    pushStyleTagsFor(iframeHTML, includedCSS);
-    const requireKernelUrl =
-        absUrl(`../static/js/require-kernel.js?v=${clientVars.randomVersionString}`);
-    iframeHTML.push(`<script type="text/javascript" src="${requireKernelUrl}"></script>`);
-    // Pre-fetch modules to improve load performance.
-    for (const module of ['ace2_inner', 'ace2_common']) {
-      const url = absUrl(`../javascripts/lib/ep_etherpad-lite/static/js/${module}.js` +
-                         `?callback=require.define&v=${clientVars.randomVersionString}`);
-      iframeHTML.push(`<script type="text/javascript" src="${url}"></script>`);
-    }
-
-    iframeHTML.push(scriptTag(`(async () => {
-      parent.parent.debugLog('Ace2Editor.init() inner frame ready');
-      const require = window.require;
-      require.setRootURI(${JSON.stringify(absUrl('../javascripts/src'))});
-      require.setLibraryURI(${JSON.stringify(absUrl('../javascripts/lib'))});
-      require.setGlobalKeyPath('require');
-
-      // intentially moved before requiring client_plugins to save a 307
-      window.Ace2Inner = require('ep_etherpad-lite/static/js/ace2_inner');
-      window.plugins = require('ep_etherpad-lite/static/js/pluginfw/client_plugins');
-      window.plugins.adoptPluginsFromAncestorsOf(window);
-
-      window.$ = window.jQuery = require('ep_etherpad-lite/static/js/rjquery').jQuery;
-
-      parent.parent.debugLog('Ace2Editor.init() waiting for plugins');
-      await new Promise((resolve, reject) => window.plugins.ensure(
-          (err) => err != null ? reject(err) : resolve()));
-      parent.parent.debugLog('Ace2Editor.init() waiting for Ace2Inner.init()');
-      const editorInfo = parent.parent.ace2EditorInfo;
-      await new Promise((resolve, reject) => window.Ace2Inner.init(
-          editorInfo, (err) => err != null ? reject(err) : resolve()));
-      parent.parent.debugLog('Ace2Editor.init() Ace2Inner.init() returned');
-      editorInfo.onEditorReady();
-    })();`));
-
-    iframeHTML.push('<style type="text/css" title="dynamicsyntax"></style>');
-
-    hooks.callAll('aceInitInnerdocbodyHead', {
-      iframeHTML,
-    });
-
-    iframeHTML.push('</head><body id="innerdocbody" class="innerdocbody" role="application" ' +
-                    'spellcheck="false">&nbsp;</body></html>');
-
-    const outerScript = `(async () => {
-      await new Promise((resolve) => { window.onload = () => resolve(); });
-      parent.debugLog('Ace2Editor.init() outer frame ready');
-      window.onload = null;
-      await new Promise((resolve) => setTimeout(resolve, 0));
-      const iframe = document.createElement('iframe');
-      iframe.name = 'ace_inner';
-      iframe.title = 'pad';
-      iframe.scrolling = 'no';
-      iframe.frameBorder = 0;
-      iframe.allowTransparency = true; // for IE
-      iframe.ace_outerWin = window;
-      document.body.insertBefore(iframe, document.body.firstChild);
-      const doc = iframe.contentWindow.document;
-      doc.open();
-      doc.write(${JSON.stringify(iframeHTML.join('\n'))});
-      doc.close();
-      parent.debugLog('Ace2Editor.init() waiting for inner frame');
-    })();`;
-
-    const outerHTML =
-        [doctype, `<html class="outer-editor outerdoc ${clientVars.skinVariants}"><head>`];
-    pushStyleTagsFor(outerHTML, includedCSS);
-
-    // bizarrely, in FF2, a file with no "external" dependencies won't finish loading properly
-    // (throbs busy while typing)
-    const pluginNames = pluginUtils.clientPluginNames();
-    outerHTML.push(
-        '<style type="text/css" title="dynamicsyntax"></style>',
-        scriptTag(outerScript),
-        '</head>',
-        '<body id="outerdocbody" class="outerdocbody ', pluginNames.join(' '), '">',
-        '<div id="sidediv" class="sidediv"><!-- --></div>',
-        '<div id="linemetricsdiv">x</div>',
-        '</body></html>');
-
-    const outerFrame = document.createElement('IFRAME');
+    const outerFrame = document.createElement('iframe');
     outerFrame.name = 'ace_outer';
     outerFrame.frameBorder = 0; // for IE
     outerFrame.title = 'Ether';
     info.frame = outerFrame;
     document.getElementById(containerId).appendChild(outerFrame);
+    const outerWindow = outerFrame.contentWindow;
 
-    const editorDocument = outerFrame.contentWindow.document;
-
+    // For some unknown reason Firefox replaces outerWindow.document with a new Document object some
+    // time between running the above code and firing the outerWindow load event. Work around it by
+    // waiting until the load event fires before mutating the Document object.
     debugLog('Ace2Editor.init() waiting for outer frame');
-    await new Promise((resolve, reject) => {
-      info.onEditorReady = (err) => err != null ? reject(err) : resolve();
-      editorDocument.open();
-      editorDocument.write(outerHTML.join(''));
-      editorDocument.close();
-    });
+    await frameReady(outerFrame);
+    debugLog('Ace2Editor.init() outer frame ready');
+
+    // This must be done after the Window's load event. See above comment.
+    const outerDocument = outerWindow.document;
+
+    // <html> tag
+    outerDocument.insertBefore(
+        outerDocument.implementation.createDocumentType('html', '', ''), outerDocument.firstChild);
+    outerDocument.documentElement.classList.add('outer-editor', 'outerdoc', ...skinVariants);
+
+    // <head> tag
+    addStyleTagsFor(outerDocument, includedCSS);
+    const outerStyle = outerDocument.createElement('style');
+    outerStyle.type = 'text/css';
+    outerStyle.title = 'dynamicsyntax';
+    outerDocument.head.appendChild(outerStyle);
+
+    // <body> tag
+    outerDocument.body.id = 'outerdocbody';
+    outerDocument.body.classList.add('outerdocbody', ...pluginUtils.clientPluginNames());
+    const sideDiv = outerDocument.createElement('div');
+    sideDiv.id = 'sidediv';
+    sideDiv.classList.add('sidediv');
+    outerDocument.body.appendChild(sideDiv);
+    const lineMetricsDiv = outerDocument.createElement('div');
+    lineMetricsDiv.id = 'linemetricsdiv';
+    lineMetricsDiv.appendChild(outerDocument.createTextNode('x'));
+    outerDocument.body.appendChild(lineMetricsDiv);
+
+    const innerFrame = outerDocument.createElement('iframe');
+    innerFrame.name = 'ace_inner';
+    innerFrame.title = 'pad';
+    innerFrame.scrolling = 'no';
+    innerFrame.frameBorder = 0;
+    innerFrame.allowTransparency = true; // for IE
+    innerFrame.ace_outerWin = outerWindow;
+    outerDocument.body.insertBefore(innerFrame, outerDocument.body.firstChild);
+    const innerWindow = innerFrame.contentWindow;
+
+    // Wait before mutating the inner document. See above comment recarding outerWindow load.
+    debugLog('Ace2Editor.init() waiting for inner frame');
+    await frameReady(innerFrame);
+    debugLog('Ace2Editor.init() inner frame ready');
+
+    // This must be done after the Window's load event. See above comment.
+    const innerDocument = innerWindow.document;
+
+    // <html> tag
+    innerDocument.insertBefore(
+        innerDocument.implementation.createDocumentType('html', '', ''), innerDocument.firstChild);
+    innerDocument.documentElement.classList.add('inner-editor', ...skinVariants);
+
+    // <head> tag
+    addStyleTagsFor(innerDocument, includedCSS);
+    const requireKernel = innerDocument.createElement('script');
+    requireKernel.type = 'text/javascript';
+    requireKernel.src =
+        absUrl(`../static/js/require-kernel.js?v=${clientVars.randomVersionString}`);
+    innerDocument.head.appendChild(requireKernel);
+    // Pre-fetch modules to improve load performance.
+    for (const module of ['ace2_inner', 'ace2_common']) {
+      const script = innerDocument.createElement('script');
+      script.type = 'text/javascript';
+      script.src = absUrl(`../javascripts/lib/ep_etherpad-lite/static/js/${module}.js` +
+                          `?callback=require.define&v=${clientVars.randomVersionString}`);
+      innerDocument.head.appendChild(script);
+    }
+    const innerStyle = innerDocument.createElement('style');
+    innerStyle.type = 'text/css';
+    innerStyle.title = 'dynamicsyntax';
+    innerDocument.head.appendChild(innerStyle);
+    const headLines = [];
+    hooks.callAll('aceInitInnerdocbodyHead', {iframeHTML: headLines});
+    const tmp = innerDocument.createElement('div');
+    tmp.innerHTML = headLines.join('\n');
+    while (tmp.firstChild) innerDocument.head.appendChild(tmp.firstChild);
+
+    // <body> tag
+    innerDocument.body.id = 'innerdocbody';
+    innerDocument.body.classList.add('innerdocbody');
+    innerDocument.body.setAttribute('role', 'application');
+    innerDocument.body.setAttribute('spellcheck', 'false');
+    innerDocument.body.appendChild(innerDocument.createTextNode('\u00A0')); // &nbsp;
+
+    debugLog('Ace2Editor.init() waiting for require kernel load');
+    await eventFired(requireKernel, 'load');
+    debugLog('Ace2Editor.init() require kernel loaded');
+    const require = innerWindow.require;
+    require.setRootURI(absUrl('../javascripts/src'));
+    require.setLibraryURI(absUrl('../javascripts/lib'));
+    require.setGlobalKeyPath('require');
+
+    // intentially moved before requiring client_plugins to save a 307
+    innerWindow.Ace2Inner = require('ep_etherpad-lite/static/js/ace2_inner');
+    innerWindow.plugins = require('ep_etherpad-lite/static/js/pluginfw/client_plugins');
+    innerWindow.plugins.adoptPluginsFromAncestorsOf(innerWindow);
+
+    innerWindow.$ = innerWindow.jQuery = require('ep_etherpad-lite/static/js/rjquery').jQuery;
+
+    debugLog('Ace2Editor.init() waiting for plugins');
+    await new Promise((resolve, reject) => innerWindow.plugins.ensure(
+        (err) => err != null ? reject(err) : resolve()));
+    debugLog('Ace2Editor.init() waiting for Ace2Inner.init()');
+    await new Promise((resolve, reject) => innerWindow.Ace2Inner.init(
+        info, (err) => err != null ? reject(err) : resolve()));
+    debugLog('Ace2Editor.init() Ace2Inner.init() returned');
     loaded = true;
     doActionsPendingInit();
     debugLog('Ace2Editor.init() done');