A quick tip that stumped me this morning:

Every now and again Firefox will not pick up a CSS change you have made - even hitting F5 / Refresh wont solve the issue. On a Windows machine using CTRL & F5 will perform a hard refresh and solve the problem.

Hitting the Mac equivilent of CMD & F5 on a Mac will instead turn Voiceover on - so to solve this you instead need to run CMD & R to perform the hard refresh.

Hope this helps!