firefox: allow overriding config of wrapped package

By moving the `cfg` variable into the wrapper arguments we are able to
override it for an already wrapped package. For example, with this
change one can have

    pkgs.firefox-devedition-bin.override {
      cfg.enableBrowserpass = true;
    }

which would otherwise be difficult to accomplish for packages having a
complicated wrapped definition in `all-packages.nix`.

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

@@ -28,12 +28,12 @@ let
     , extraPlugins ? []
     , extraNativeMessagingHosts ? []
     , gdkWayland ? false
+    , cfg ? config.${browserName} or {}
     }:
 
     assert gdkWayland -> (browser ? gtk3); # Can only use the wayland backend if gtk3 is being used
 
     let
-      cfg = config.${browserName} or {};
       enableAdobeFlash = cfg.enableAdobeFlash or false;
       ffmpegSupport = browser.ffmpegSupport or false;
       gssSupport = browser.gssSupport or false;