thunderbird, firefox, firefox-esr: set big-parallel

Firefox and Thunder time out nearly everytime. Hopefully giving them
more resources will prevent this from happening.

(cherry picked from commit 29f1edf2f3 / PR #105345)
Too many time-outs on 20.09, so let's try this also here.

pkgs/applications/networking/browsers/firefox/common.nix

@@ -358,4 +358,6 @@ stdenv.mkDerivation ({
 
   # on aarch64 this is also required
   dontUpdateAutotoolsGnuConfigScripts = true;
+
+  requiredSystemFeatures = [ "big-parallel" ];
 })

pkgs/applications/networking/mailreaders/thunderbird/default.nix

@@ -321,6 +321,8 @@ stdenv.mkDerivation rec {
       gnugrep curl runtimeShell;
   };
 
+  requiredSystemFeatures = [ "big-parallel" ];
+
   meta = with stdenv.lib; {
     description = "A full-featured e-mail client";
     homepage = "https://www.thunderbird.net";