I've seen some similar posts on the stack, but I can't find a working solution for me. In our project we are using jQuery to create a loading screen. On Opera and Opera Gaming this feature works great, but in firefox nightly (if that matters) it does not. Here the function in question:

 let loadingScreen = function () {
      $(window).on("load", function () {
          $(".loader-wrapper").fadeOut("slow");
      });
  };

Link to the entire repository: https://github.com/radekaadek/szukaj-id