--- a/devtools/client/devtools-startup.js
+++ b/devtools/client/devtools-startup.js
@@ -8,17 +8,17 @@
  * core modules like 'devtools-browser.js' that hooks the browser windows
  * and ensure setting up tools.
  *
  * Be careful to lazy load dependencies as much as possible.
  **/
 
 "use strict";
 
-const { interfaces: Ci, utils: Cu } = Components;
+const { classes: Cc, interfaces: Ci, utils: Cu } = Components;
 const kDebuggerPrefs = [
   "devtools.debugger.remote-enabled",
   "devtools.chrome.enabled"
 ];
 const { XPCOMUtils } = Cu.import("resource://gre/modules/XPCOMUtils.jsm", {});
 XPCOMUtils.defineLazyModuleGetter(this, "Services", "resource://gre/modules/Services.jsm");
 
 function DevToolsStartup() {}
@@ -119,19 +119,38 @@ DevToolsStartup.prototype = {
     }
     return remoteDebuggingEnabled;
   },
 
   handleDebuggerFlag: function (cmdLine) {
     if (!this._isRemoteDebuggingEnabled()) {
       return;
     }
+
+    let devtoolsThreadResumed = false;
+    let pauseOnStartup = cmdLine.handleFlag("wait-for-jsdebugger", false);
+    if (pauseOnStartup) {
+      let observe = function (subject, topic, data) {
+        devtoolsThreadResumed = true;
+        Services.obs.removeObserver(observe, "devtools-thread-resumed");
+      };
+      Services.obs.addObserver(observe, "devtools-thread-resumed", false);
+    }
+
     const { BrowserToolboxProcess } = Cu.import("resource://devtools/client/framework/ToolboxProcess.jsm", {});
     BrowserToolboxProcess.init();
 
+    if (pauseOnStartup) {
+      // Spin the event loop until the debugger connects.
+      let thread = Cc["@mozilla.org/thread-manager;1"].getService().currentThread;
+      while (!devtoolsThreadResumed) {
+        thread.processNextEvent(true);
+      }
+    }
+
     if (cmdLine.state == Ci.nsICommandLine.STATE_REMOTE_AUTO) {
       cmdLine.preventDefault = true;
     }
   },
 
   /**
    * Handle the --start-debugger-server command line flag. The options are:
    * --start-debugger-server
@@ -198,16 +217,19 @@ DevToolsStartup.prototype = {
     if (cmdLine.state == Ci.nsICommandLine.STATE_REMOTE_AUTO) {
       cmdLine.preventDefault = true;
     }
   },
 
   /* eslint-disable max-len */
   helpInfo: "  --jsconsole        Open the Browser Console.\n" +
             "  --jsdebugger       Open the Browser Toolbox.\n" +
+            "  --wait-for-jsdebugger Spin event loop until JS debugger connects.\n" +
+            "                     Enables debugging (some) application startup code paths.\n" +
+            "                     Only has an effect when `--jsdebugger` is also supplied.\n" +
             "  --devtools         Open DevTools on initial load.\n" +
             "  --start-debugger-server [ws:][ <port> | <path> ] Start the debugger server on\n" +
             "                     a TCP port or Unix domain socket path. Defaults to TCP port\n" +
             "                     6000. Use WebSocket protocol if ws: prefix is specified.\n",
   /* eslint-disable max-len */
 
   classID: Components.ID("{9e9a9283-0ce9-4e4a-8f1c-ba129a032c32}"),
   QueryInterface: XPCOMUtils.generateQI([Ci.nsICommandLineHandler]),

--- a/devtools/server/actors/chrome.js
+++ b/devtools/server/actors/chrome.js
@@ -45,17 +45,28 @@ function ChromeActor(connection) {
   // Defines the default docshell selected for the tab actor
   let window = Services.wm.getMostRecentWindow(DebuggerServer.chromeWindowType);
 
   // Default to any available top level window if there is no expected window
   // (for example when we open firefox with -webide argument)
   if (!window) {
     window = Services.wm.getMostRecentWindow(null);
   }
-  // On xpcshell, there is no window/docshell
+
+  // We really want _some_ window at least, so fallback to the hidden window if
+  // there's nothing else (such as during early startup).
+  if (!window) {
+    try {
+      window = Services.appShell.hiddenDOMWindow;
+    } catch (e) {
+      // On XPCShell, the above line will throw.
+    }
+  }
+
+  // On XPCShell, there is no window/docshell
   let docShell = window ? window.QueryInterface(Ci.nsIInterfaceRequestor)
                                 .getInterface(Ci.nsIDocShell)
                         : null;
   Object.defineProperty(this, "docShell", {
     value: docShell,
     configurable: true
   });
 }

--- a/devtools/server/actors/script.js
+++ b/devtools/server/actors/script.js
@@ -1015,18 +1015,18 @@ const ThreadActor = ActorClassWithSpec(t
         this._options.pauseOnExceptions = aRequest.pauseOnExceptions;
         this._options.ignoreCaughtExceptions = aRequest.ignoreCaughtExceptions;
         this.maybePauseOnExceptions();
         this._maybeListenToEvents(aRequest);
       }
 
       let packet = this._resumed();
       this._popThreadPause();
-      // Tell anyone who cares of the resume (as of now, that's the xpcshell
-      // harness)
+      // Tell anyone who cares of the resume (as of now, that's the xpcshell harness and
+      // devtools-startup.js when handling the --wait-for-jsdebugger flag)
       if (Services.obs) {
         Services.obs.notifyObservers(this, "devtools-thread-resumed", null);
       }
       return packet;
     }, error => {
       return error instanceof Error
         ? { error: "unknownError",
             message: DevToolsUtils.safeErrorString(error) }