I can get a global rule stating
* #user_tools { position:absolute !important ; }
to work on itch, but it may break other sites.
Viewing post in Game pages trigger my migraines
Check your file in an editor with syntax highlighting. You have bugs in there and firefox does not tell you this.
The addons used to integrate user styles have syntax checking and can apply it on the fly, without restarting firefox. I used Stylus 1.5.33 on Chrome and that is available for firefox as well.
If a global setting works, the namespaces are screwed up somehow. You probably do not even see userContent.css in the inspecor, do you. Because ff does not know that it has to apply the file to the site.