--- a/devtools/client/framework/ToolboxProcess.jsm
+++ b/devtools/client/framework/ToolboxProcess.jsm
@@ -244,28 +244,32 @@ BrowserToolboxProcess.prototype = {
   /**
    * Creates and initializes the profile & process for the remote debugger.
    */
   _create: function () {
     dumpn("Initializing chrome debugging process.");
 
     let command = Services.dirsvc.get("XREExeF", Ci.nsIFile).path;
 
-    let xulURI = `${DBG_XUL}?port=${this.port}`;
-    if (this._options.addonID) {
-      xulURI += `&addonID=${this._options.addonID}`;
-    }
-
     dumpn("Running chrome debugging process.");
     let args = [
       "-no-remote",
       "-foreground",
       "-profile", this._dbgProfilePath,
-      "-chrome", xulURI
+      "-chrome", DBG_XUL
     ];
+    let environment = {
+      // Disable safe mode for the new process in case this was opened via the
+      // keyboard shortcut.
+      MOZ_DISABLE_SAFE_MODE_KEY: "1",
+      MOZ_BROWSER_TOOLBOX_PORT: String(this.port),
+    };
+    if (this._options.addonID) {
+      environment.MOZ_BROWSER_TOOLBOX_ADDONID = String(this._options.addonID);
+    }
 
     // During local development, incremental builds can trigger the main process
     // to clear its startup cache with the "flag file" .purgecaches, but this
     // file is removed during app startup time, so we aren't able to know if it
     // was present in order to also clear the child profile's startup cache as
     // well.
     //
     // As an approximation of "isLocalBuild", check for an unofficial build.
@@ -273,21 +277,17 @@ BrowserToolboxProcess.prototype = {
       args.push("-purgecaches");
     }
 
     this._dbgProcessPromise = Subprocess.call({
       command,
       arguments: args,
       environmentAppend: true,
       stderr: "stdout",
-      environment: {
-        // Disable safe mode for the new process in case this was opened via the
-        // keyboard shortcut.
-        MOZ_DISABLE_SAFE_MODE_KEY: "1",
-      },
+      environment,
     }).then(proc => {
       this._dbgProcess = proc;
 
       this._telemetry.toolOpened("jsbrowserdebugger");
 
       dumpn("Chrome toolbox is now running...");
       this.emit("run", this);
 

--- a/devtools/client/framework/toolbox-process-window.js
+++ b/devtools/client/framework/toolbox-process-window.js
@@ -29,32 +29,35 @@ var Prefs = new PrefsHelper("devtools.de
 });
 
 var gToolbox, gClient;
 
 var connect = Task.async(function* () {
   window.removeEventListener("load", connect);
 
   // Initiate the connection
-  const params = new URLSearchParams(window.location.search);
+  let env = Components.classes["@mozilla.org/process/environment;1"]
+    .getService(Components.interfaces.nsIEnvironment);
+  let port = env.get("MOZ_BROWSER_TOOLBOX_PORT");
+  let addonID = env.get("MOZ_BROWSER_TOOLBOX_ADDONID");
 
-  // A port needs to be passed in from the query string, for instance:
-  // `./mach run -chrome chrome://devtools/content/framework/toolbox-process-window.xul?port=6080`
-  if (!params.get("port")) {
-    throw new Error("Must specify a port on the query string");
+  // A port needs to be passed in from the environment, for instance:
+  //    MOZ_BROWSER_TOOLBOX_PORT=6080 ./mach run -chrome \
+  //      chrome://devtools/content/framework/toolbox-process-window.xul
+  if (!port) {
+    throw new Error("Must pass a port in an env variable with MOZ_BROWSER_TOOLBOX_PORT");
   }
 
   let transport = yield DebuggerClient.socketConnect({
     host: Prefs.chromeDebuggingHost,
-    port: params.get("port"),
+    port,
     webSocket: Prefs.chromeDebuggingWebSocket,
   });
   gClient = new DebuggerClient(transport);
   yield gClient.connect();
-  let addonID = params.get("addonID");
 
   if (addonID) {
     let { addons } = yield gClient.listAddons();
     let addonActor = addons.filter(addon => addon.id === addonID).pop();
     let isTabActor = addonActor.isWebExtension;
     openToolbox({form: addonActor, chrome: true, isTabActor});
   } else {
     let response = yield gClient.getProcess();