wrapFirefox: fix #25505 gtk3 file dialogs

Crash + icons.

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

···
     gtk = gtk2;
     inherit nspr;
     inherit ffmpegSupport;
-  };
+  } // lib.optionalAttrs gtk3Support { inherit gtk3; };
 
 } // overrides)

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

···
     ];
   };
 
-  buildInputs = [makeWrapper] ++ lib.optionals (!ffmpegSupport) gst-plugins;
+  buildInputs = [makeWrapper]
+    ++ lib.optional (!ffmpegSupport) gst-plugins
+    ++ lib.optional (browser ? gtk3) browser.gtk3;
 
   buildCommand = ''
     if [ ! -x "${browser}/bin/${browserName}" ]
···
         --prefix-contents PATH ':' "$(filterExisting $(addSuffix /extra-bin-path $plugins))" \
         --suffix PATH ':' "$out/bin" \
         --set MOZ_APP_LAUNCHER "${browserName}${nameSuffix}" \
-        ${lib.optionalString (!ffmpegSupport) ''--prefix GST_PLUGIN_SYSTEM_PATH : "$GST_PLUGIN_SYSTEM_PATH"''}
+        ${lib.optionalString (!ffmpegSupport)
+            ''--prefix GST_PLUGIN_SYSTEM_PATH : "$GST_PLUGIN_SYSTEM_PATH"''
+        + lib.optionalString (browser ? gtk3)
+            ''--prefix XDG_DATA_DIRS : "$GSETTINGS_SCHEMAS_PATH" \
+              --suffix XDG_DATA_DIRS : '${gnome3.defaultIconTheme}/share'
+            ''
+         }
 
     if [ -e "${browser}/share/icons" ]; then
         mkdir -p "$out/share"