I'm currently using the latest versions of both firefox and flash.
I can play flash videoes and games fine, but as soon as i go full screen, the video lags, and you can hear the audio stuttering and the mouse is a little unresponsive.
Trying to watch HQ videoes in full screen is totally out of the question and lags really badly.
I've tried cleaning cache, turning off hardware acceleration, and cleaning flash files, but it still happens.
Update your extensions. about:addons
Update all your filter subscriptions in Adblock Plus
with Right click on ABP button > Filter Preferences > Action > update filter
Reboot your system and bring up Firefox
I suspect it was the filter subscriptions and/or the reboot that fixed things on my system.
I updated anything else that needed to be, and did that update filter at Adblock Plus.
It's... a bit weird.
When i try at youtube, it doesn't lag.
When i switch to HD video it lags a tiny bit, and less when i let the bar disappear, but if i right click to display options it gets really really laggy.
On other sites it lags and skips frames even though the video is low quality.
Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance).
Do not click the Reset button on the Safe mode start window or otherwise make changes.