Bug 855928 - Add a telemetry probe to report default desktop browser status. r=gavin

browser/components/nsBrowserGlue.js

@@ -580,9 +580,17 @@ BrowserGlue.prototype = {
           (ss.sessionType == Ci.nsISessionStartup.RECOVER_SESSION);
       }
       catch (ex) { /* never mind; suppose SessionStore is broken */ }
-      if (shouldCheck &&
-          !shell.isDefaultBrowser(true, false) &&
-          !willRecoverSession) {
+
+      let isDefault = shell.isDefaultBrowser(true, false); // startup check, check all assoc
+      try {
+        // Report default browser status on startup to telemetry
+        // so we can track whether we are the default.
+        Services.telemetry.getHistogramById("BROWSER_IS_USER_DEFAULT")
+                          .add(isDefault);
+      }
+      catch (ex) { /* Don't break the default prompt if telemetry is broken. */ }
+
+      if (shouldCheck && !isDefault && !willRecoverSession) {
         Services.tm.mainThread.dispatch(function() {
           var win = this.getMostRecentBrowserWindow();
           var brandBundle = win.document.getElementById("bundle_brand");

toolkit/components/telemetry/Histograms.json

@@ -3569,5 +3569,9 @@
     "high": "10000000",
     "n_buckets": 100,
     "description": "How long has the developer toolbar been active (seconds)"
+  },
+  "BROWSER_IS_USER_DEFAULT": {
+    "kind": "boolean",
+    "description": "The result of the startup default desktop browser check."
   }
 }