I'm not sure what exactly is going on, but while reading a long pdf in full screen, without touching anything, sometimes I'm kicked out of full screen. This is the pdf I'm reading.  It's happened now twice, seemingly randomly, as I was reading (no user input). I've simply clicked the "Switch to Presentation Mode" button in the top right. Firefox itself is not "lion full screen" before making the pdf full screen. I'm on OSX 10.10.5 using FF 44.0a2 (2015-12-14).  http://free-electrons.com/doc/training/android/android-slides.pdf
I'm seeing this on youtube now too, I think the fullscreen functionality itself is busted.
Component: PDF Viewer → General
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