Same Problem here, I have tried About:config Browser-session store-interval changed to 300000 but it only kind of worked then I changed it back to 15000 ..it seems as though 720p and 1080p video in full screen plays faster then the video can buffer. and or the video will stop mid-play as if the video was over yet still displays 78% of the video has yet to be played. I closed firefox and as an experiment I opened the same video in IE9 and no problems at all with buffering no problems in full screen mode at 1080p at all. only With IE9 I am Forced to watch the 30 second Advertisements before any video can play. this is why I like Firefox. NO AD's ad Block Plus really does a great job. I forgot to mention I am Running Windows 7 32 bit. XfX Radeon HD 5770 Graphics card. AMD athlon x4 Processor. 8Gb DDR2. and NO I AM NOT GOING TO USE windows 8.. this is a PC not a cell phone or an Ipad.

I've had this problem since Flash version 11.4 and it is still a problem in 11.5.

I can't watch 720p and 1080p videos any more with Firefox.

This is a blatant issue with Firefox. Disabling addons does not fix the issue. The same videos play fine in HD quality in Chrome.

I have reinstalled Firefox and Flash over a dozen times (I'm not exaggerating), followed several troubleshooting guides here on this support site, cleared Cache and every other install file and profile file several times and spent hours Googling. Nothing worked.

The only solution to fix this right now, that works, is to install Flash 11.3 or 11.2 from the Adobe archive here: http://helpx.adobe.com/flash-player/kb/archived-flash-player-versions.html

Or just install Google Chrome and the Adblock for it to get rid of the ads.

I believe it to be a Firefox and Flash Conflict. that either Mozilla or Adobe have to address... hopefully soon. if Mozilla staff even read these threads.