Firefox for Windows users have had full screen transitions forever, so I'm surprised you're just getting it now. There is a pair of hidden settings to make the transition almost instant (I can't say it's actually instant, but it's barely noticeable). Here's how you change them in Firefox for Windows (assuming it works the same way):
(A) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.
More info on about:config: Configuration Editor for Firefox. Please keep in mind that changes made through this back door aren't fully supported and aren't guaranteed to continue working in the future.
(B) In the search box in the page, type or paste full-screen-api.transition-duration and pause while the list is filtered
Firefox should list two preferences, one ending with enter and one ending with leave.
(C) Double-click the full-screen-api.transition-duration.enter preference to display an editing field, and change the value to 0 0 -- that's zero space zero -- then press Enter or click the blue check mark button to save the change.
(D) Double-click the full-screen-api.transition-duration.leave preference to display an editing field, and change the value to 0 0 -- that's zero space zero -- then press Enter or click the blue check mark button to save the change.
Then give it a test. Success?
If not, hopefully a Mac person can chime in.
