That element behaves differently for different pixel widths (see at bottom of post). If you have big enough zoom it will scroll away with the page.
And that style will anchor it to the page. If it does not for you, your style addon does not work properly. Try the code with display to see, if you can disable the element, if not you know for sure, that your addon does not work.
You can also inspect the element to see what styles are active on this. If too big , the style at the bottom is active and the fixed style gets crossed out.
@-moz-document domain("itch.io") { #user_tools { position:absolute; } }
@-moz-document domain("itch.io") { #user_tools { display:none; } }
@media (max-width: 1300px) .user_tools { position: absolute; }