They tried a fix and it broke YouTube. So it seems like a reasonable fear. https://bugzilla.mozilla.org/show_bug.cgi?id=349259#c54
> The patch ... is literally to remove "!important" from one CSS file
It isn't a simple one line fix, as Firefox developer kindly explained. https://bugzilla.mozilla.org/show_bug.cgi?id=349259#c76
>That patch would not be welcome.
They describe what work would be required (and where the changes would likely be) for the patch to be accepted in the bug. https://bugzilla.mozilla.org/show_bug.cgi?id=349259#c80