I experienced the problem you are describing on my home computer too. Flash video playback on any site would cause Firefox to become unresponsive as long as the video was in view. I had to shut down Firefox through Task Manager several times because none of the buttons would respond to the mouse clicks. Video in IE9 appeared to play okay, though I didn't test it much.
I first noticed problem about a month ago, shortly after I updated my GTX460 video card driver from version 301.42 to 306.02 beta. I though the issue would go away when I installed the newest stable release 306.23 but it didn't. I even updated to Flash 11.5 beta just to see if it would help.
Today I got fed up and was about to post on this forum but thought I'd do some troubleshooting first. I uninstalled the nVidia driver from the control panel, rebooted, then re-installed the old 301.42. Flash appeared to be working fine again, and so I updated the driver again to the latest version and I am all seems to be fixed now.
I have been testing around for a couple hours, loading videos from different sites, in multiple tabs etc. and I cannot cause Firefox to misbehave anymore. So far so good, fingers crossed.
(Windows 7 Ultimate 64bit / AMD Phenom II 940 / 8GB DDR2 / Nvidia Geforce GTX460)
Message was edited by: MikeLevinski (I meant to reply to the the person with the browser slowdowns problem)
The original thread has become too long to be useful. I'm branching out the individual issues so that we can better track and respond to them.