wrapFirefox: fix icon linking for Thunderbird

Firefox has its icons in
`/lib/firefox-bin-*/browser/chrome/icons/default` while Thunderbird has
`/lib/thunderbird/chrome/icons/default`.

Fixes https://github.com/NixOS/nixpkgs/pull/143942#issuecomment-963418080.

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

@@ -290,7 +290,7 @@ let
         else
             for res in 16 32 48 64 128; do
             mkdir -p "$out/share/icons/hicolor/''${res}x''${res}/apps"
-            icon=( "${browser}/lib/"*"/browser/chrome/icons/default/default''${res}.png" )
+            icon=$( find "${browser}/lib/" -name "default''${res}.png" )
               if [ -e "$icon" ]; then ln -s "$icon" \
                 "$out/share/icons/hicolor/''${res}x''${res}/apps/${applicationName}.png"
               fi