Maybe disable hardware acceleration in Firefox.

• Firefox > Preferences > Advanced > General > Browsing: "Use hardware acceleration when available"

Unfortunately that did not make any difference. Still the same native Lion fullscreen I'm afraid.

This is a major problem to me as well. First it was Safari, obviously. Next in line was Chrome with their OSx Lion Full Screen integration... lastly Firefox!

What browser with decent HTML5 performance am I to use in a multi monitor setup when I need full screen?

The way Firefox handled full screen on OSx Lion is the main reason I was using Firefox.

I too wish this could be turned off. I don't use multiple monitors, but the old method of fullscreen was much more efficient, especially since FF 13 when they made new windows mimic the currently active one (meaning I don't have to press the fullscreen button every time I open a new window).

Now, in 14, every new window I create while in fullscreen is a new Space in mission control that I have to worry about. I don't really feel like getting into all of it, but there are several small issues that this creates that, when combined, lead to a very unpleasant browsing experience.

rtyrty, you asked, Is it possible to disable the support in some way to actually have useful fullscreen support again, i.e. the way it worked before v14.0.1?

This question also came up here: https://support.mozilla.org/questions/932384

Related bug report: Bug 740148 - [10.7] Allow hiding toolbars for presentations.

I've built a simple addon that does just that: reverts the old pre-14 Fullscreen behavior. It isn't yet reviewed, so wouldn't show in searches until approved.

Here's the direct link: https://addons.mozilla.org/en-us/firefox/addon/old-lion-fullscreen/. Bugreports are welcome!

Thank you so much arp. Your add-on works perfectly. I left you a 5 star review. If I see any bugs I will be sure to let you know.

Arp, thanks a million for taking time to fix this problem! Firefox, Safari and Chrome are useless for my portfolio website I designed to present my work. It's a website that uses two screens simultaneously. I immediately installed your add-on as soon as I read your reply. There is just one small bug when I use it in Mountain Lion (not sure why it happens). I have two Firefox windows open, one on each monitor. On both of them I set them to full screen, using your add-on. The main monitor works flawlessly, the second goes to full screen but leaves a blank space on top. See the attached image for an example (I hover the white space in the picture, that's why the tab bar appears). I just made a clean Mountain Lion install yesterday and your add-on is the first I installed on Firefox so I can't think of anything interfering. My second monitor is actually an Android phone acting as a wifi connected monitor (using the app iDisplay) but it was working fine for me before all this full screen api non-sense. So the white bar shouldn't be cause by that according to me.

jip-hop, thanks for checking the extension! I suppose you see something that looks like this old issue: https://bugzilla.mozilla.org/show_bug.cgi?id=551235. I'll check this once I get to my second monitor, but my immediate thought is that the problem is related to how Firefox itself behaves on Mac OS X.

Yes, that looks like the exact same problem. However under Snow Leopard and Firefox 13 (if I remember correctly), I didn't have such problems with white bars on my second monitor. That means that Mozilla and Apple DID work well together at some point in history :P So we know the white bars don't HAVE to be there... I'm looking forward to your second monitor as well and hopefully you'll find out a fix for this :) thanks again, as an add-on causing a little white space is already a big improvement on not being able to use my portfolio at all any more!

So, I've been able to reproduce the issue -- it's intermittent and while some windows have that white strip on the top, some of them work just fine.

The bad news is that this strip is caused by Firefox itself, it's just the old bug with browser's own Fullscreen mode that reappeared.

In my case, it was enough to hit Cmd-Shift-F several times and move the window between desktops to get an absolutely normal window. Restarting Firefox also opened all its saved windows without a stripe.

I'm also using Tab Utilities extension (https://addons.mozilla.org/en-US/firefox/addon/tab-utilities/) which seems to somehow affect the way the top of the browser window is handled, so you may try using it as well.