fixes firefox bug so css custom prop is the correct value

opentech/static_src/src/javascript/main.js

@@ -150,10 +150,9 @@
     });
 
     // Get the header and admin bar height and set custom prop with value
-    (() => {
+    $(window).on('load', function () {
         const headerHeight = $('.header').outerHeight();
         const adminbarHeight = $('.admin-bar').outerHeight();
         document.documentElement.style.setProperty('--header-admin-height', headerHeight + adminbarHeight + 'px');
-    })();
-
+    });
 })(jQuery);