diff -urN mozilla-release.orig/browser/app/profile/firefox.js mozilla-release/browser/app/profile/firefox.js
--- mozilla-release.orig/browser/app/profile/firefox.js	2015-07-03 13:03:04.894795120 +0300
+++ mozilla-release/browser/app/profile/firefox.js	2015-07-03 13:21:53.836449297 +0300
@@ -271,7 +271,7 @@
 #endif
 
 // At startup, check if we're the default browser and prompt user if not.
-pref("browser.shell.checkDefaultBrowser", true);
+pref("browser.shell.checkDefaultBrowser", false);
 pref("browser.shell.shortcutFavicons",true);
 pref("browser.shell.mostRecentDateSetAsDefault", "");