I'm going to test gtk_show_uri() (and the _on_window variant) on Windows, maybe it works for file:// and http:// URIs even without GVFS what should be well enough for opening URLs in the browser.
Great, don't forget to test URLs with `#foo` on them in case GTK just uses `ShellExecute` internally.
The PR sounds great, but I'm not sure about your last point though since it will break firefox for those where it does actually work.