I did not know this can be done. Good to know. Anyways. Firefox seems to ignore your file. It would do a display none, but the position attribute is not used from the file, but from the site css. Use important. That works. And use inspect button to look the element. You can see wich css is used.
(Edit. I read the discussion in github. I can not tell you why your firefox does not do it. I can only tell you, my firefox ignored it, till i wrote important. And again, use the inbuilt inspect button. It should look like the screenshot.)
@-moz-document domain("itch.io") { #user_tools { position:absolute !important ; } }