--- a/devtools/server/actors/webbrowser.js
+++ b/devtools/server/actors/webbrowser.js
@@ -1465,16 +1465,22 @@ TabActor.prototype = {
     if ((typeof options.serviceWorkersTestingEnabled !== "undefined") &&
         (options.serviceWorkersTestingEnabled !==
          this._getServiceWorkersTestingEnabled())) {
       this._setServiceWorkersTestingEnabled(
         options.serviceWorkersTestingEnabled
       );
     }
 
+    if ((typeof options.customUserAgent !== "undefined") &&
+         options.customUserAgent !== this._getCustomUserAgent()) {
+      this._setCustomUserAgent(options.customUserAgent);
+      reload = true;
+    }
+
     // Reload if:
     //  - there's an explicit `performReload` flag and it's true
     //  - there's no `performReload` flag, but it makes sense to do so
     let hasExplicitReloadFlag = "performReload" in options;
     if ((hasExplicitReloadFlag && options.performReload) ||
        (!hasExplicitReloadFlag && reload)) {
       this.onReload();
     }
@@ -1483,16 +1489,17 @@ TabActor.prototype = {
   /**
    * Opposite of the _toggleDevToolsSettings method, that reset document state
    * when closing the toolbox.
    */
   _restoreDocumentSettings: function () {
     this._restoreJavascript();
     this._setCacheDisabled(false);
     this._setServiceWorkersTestingEnabled(false);
+    this._restoreUserAgent();
   },
 
   /**
    * Disable or enable the cache via docShell.
    */
   _setCacheDisabled: function(disabled) {
     let enable =  Ci.nsIRequest.LOAD_NORMAL;
     let disable = Ci.nsIRequest.LOAD_BYPASS_CACHE |
@@ -1566,16 +1573,48 @@ TabActor.prototype = {
       return null;
     }
 
     let windowUtils = this.window.QueryInterface(Ci.nsIInterfaceRequestor)
                                  .getInterface(Ci.nsIDOMWindowUtils);
     return windowUtils.serviceWorkersTestingEnabled;
   },
 
+  _previousCustomUserAgent: null,
+
+  /**
+   * Return custom user agent.
+   */
+  _getCustomUserAgent: function() {
+    if (!this.docShell) {
+      // The tab is already closed.
+      return null;
+    }
+    return this.docShell.customUserAgent;
+  },
+
+  /**
+   * Sets custom user agent for the current tab
+   */
+  _setCustomUserAgent: function(userAgent) {
+    if (this._previousCustomUserAgent === null) {
+      this._previousCustomUserAgent = this.docShell.customUserAgent;
+    }
+    this.docShell.customUserAgent = userAgent;
+  },
+
+  /**
+   * Restore the user agent, before the actor modified it
+   */
+  _restoreUserAgent: function() {
+    if (this._previousCustomUserAgent !== null) {
+      this.docShell.customUserAgent = this._previousCustomUserAgent;
+    }
+  },
+
   /**
    * Prepare to enter a nested event loop by disabling debuggee events.
    */
   preNest: function BTA_preNest() {
     if (!this.window) {
       // The tab is already closed.
       return;
     }