Every time. Dunno if it's me or not. Running on a Mac if that makes a difference.
What version of OS X and Safari are you using. I use Safari almost as much as Chrome and I am not seeing this issue.
I have a different issue but it's similar. When I hit fullscreen, I can't click on my other monitor without it sizing down the window. Is there a reason/fix for this?
Edit: Firefox 19.0, Win7 64bit.
We use the native browser fullscreen API and that is how Firefox treats fullscreen. Each browser treats fullscreen a little differently. From what I understand browsers have decided that Fullscreen hijacking like what you are describing and what Flash has done for years is considered a security risk. I know not a great answer but that is where we are at right now. I suggest trying windowed mode and making the window as large as possible.
When I click full screen on my mac (I have my browser on a projector set up as the secondary screen. 10.7.something, Lion) the screen slides over into a screen somehow existing to the right of my projector, that is I have the mac screen, projector, and then a separate weird non-screen to the right of that again.. And it's blank, there are no icons or anything, just the gray woven fabric like background.. Have to press escape to get back to the correct desktop and my browser.. Needless to say, this is odd.. Never had it on any other video streamything either.