--- a/devtools/client/responsivedesign/responsivedesign.jsm
+++ b/devtools/client/responsivedesign/responsivedesign.jsm
@@ -890,17 +890,19 @@ ResponsiveUI.prototype = {
 
   changeUA: function() {
     var value = this.userAgentInput.value;
     if (value !== "") {
       this.userAgentInput.setAttribute("attention", "true");
     } else {
       this.userAgentInput.removeAttribute("attention", "true");
     }
-    this.tabClient.reconfigure({customUserAgent: value});
+    this.tabClient.reconfigure({customUserAgent: value}, () => {
+      ResponsiveUIManager.emit("userAgentChanged", { tab: this.tab });
+    });
   },
 
   /**
    * Change the size of the browser.
    *
    * @param aWidth width of the browser.
    * @param aHeight height of the browser.
    */

--- a/devtools/client/responsivedesign/test/browser_responsiveui_customuseragent.js
+++ b/devtools/client/responsivedesign/test/browser_responsiveui_customuseragent.js
@@ -15,42 +15,44 @@ add_task(function*() {
   let selectedTab = gBrowser.selectedTab;
 
   let mgrOn = once(mgr, "connectedToServer");
   mgr.toggle(window, selectedTab);
   yield mgrOn;
   yield testUserAgent(DEFAULT_UA);
 
   info("Setting UA to " + CHROME_UA);
-  setUserAgent(CHROME_UA);
+  yield setUserAgent(CHROME_UA);
   yield testUserAgent(CHROME_UA);
 
   info("Resetting UA");
-  setUserAgent("");
+  yield setUserAgent("");
   yield testUserAgent(DEFAULT_UA);
 
   info("Setting UA to " + CHROME_UA);
-  setUserAgent(CHROME_UA);
+  yield setUserAgent(CHROME_UA);
   yield testUserAgent(CHROME_UA);
 
   info("Closing responsive mode");
   let mgrOff = once(mgr, "off");
   mgr.toggle(window, selectedTab);
   yield mgrOff;
   yield testUserAgent(DEFAULT_UA);
 });
 
-function setUserAgent(ua) {
+function* setUserAgent(ua) {
   let mgr = ResponsiveUI.ResponsiveUIManager;
   let instance = mgr.getResponsiveUIForTab(gBrowser.selectedTab);
   let input = instance.userAgentInput;
 
   input.focus();
   input.value = ua;
+  let onUAChanged = once(mgr, "userAgentChanged");
   input.blur();
+  yield onUAChanged;
 
   if (ua !== "") {
     ok(input.hasAttribute("attention"), "UA input should be highlighted");
   } else {
     ok(!input.hasAttribute("attention"), "UA input shouldn't be highlighted");
   }
 }