Merge pull request #24787 from abbradar/gtk3-firefox

GTK3 by default in Firefox and Thunderbird

pkgs/top-level/all-packages.nix

···
   inherit (callPackages ../applications/networking/browsers/firefox {
     inherit (gnome2) libIDL;
     libpng = libpng_apng;
-    enableGTK3 = false;
+    enableGTK3 = true;
     python = python2;
     gnused = gnused_422;
   }) firefox-unwrapped firefox-esr-unwrapped;
···
   thunderbird = callPackage ../applications/networking/mailreaders/thunderbird {
     inherit (gnome2) libIDL;
     libpng = libpng_apng;
+    enableGTK3 = true;
   };
 
   thunderbird-bin = callPackage ../applications/networking/mailreaders/thunderbird-bin {