buildMozillaMach: set geo.provider.network.url for new profiles.

Use Mozilla Location Service as geolocation provider for new profiles,
since our Google API key does not seem to work for geolocation at this
time.

Related: https://github.com/NixOS/nixpkgs/issues/173758

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

@@ -180,6 +180,18 @@ let
     };
   });
 
+  defaultPrefs = {
+    "geo.provider.network.url" = {
+      value = "https://location.services.mozilla.com/v1/geolocate?key=%MOZILLA_API_KEY%";
+      reason = "Use MLS by default for geolocation, since our Google API Keys are not working";
+    };
+  };
+
+  defaultPrefsFile = pkgs.writeText "nixos-default-prefs.js" (lib.concatStringsSep "\n" (lib.mapAttrsToList (key: value: ''
+    // ${value.reason}
+    pref("${key}", ${builtins.toJSON value.value});
+  '') defaultPrefs));
+
 in
 
 buildStdenv.mkDerivation ({
@@ -458,6 +470,7 @@ buildStdenv.mkDerivation ({
   postInstall = ''
     # Install distribution customizations
     install -Dvm644 ${distributionIni} $out/lib/${binaryName}/distribution/distribution.ini
+    install -Dvm644 ${defaultPrefsFile} $out/lib/${binaryName}/browser/defaults/preferences/nixos-default-prefs.js
 
   '' + lib.optionalString buildStdenv.isLinux ''
     # Remove SDK cruft. FIXME: move to a separate output?