It is possible that there are several ways to create the glitch on different systems. But I know that in my personal experience, it only happens when the number of total flash videos amongst the Firefox tabs is greater than fifteen. This has held true on both my Dell PC w/ Windows XP, and my HP Laptop w/ Windows Vista. FYI: I don't know if fifteen is truly the upper limit, since on rare occasion the browser seems to survive, so maybe it's more like the point where the overall likelihood of trouble shoots up dramatically.
SOLUTION: Avoid loading any web pages that contain more than 15 videos, and be careful about having too many tabs opened because for example... if you've got one tab with 12 flash vids and 8 additional tabs with 1 flash video each, the cumulative total will exceed 15 and nothing will play.
FOR DEBUGGERS: Here's an example of a web page that's extremely flash-heavy and likely to cause a freeze-up glitch.
http://my.barackobama.com/page/communit ... maroadblogToday is August 31st, 2008, and as of right now, the "Obama Road Blog" page currently has 22 embedded flash objects (a mixture of flickr slide shows and youtube videos). If that flash-heavy blog is the only tab in my firefox browser and I wait for it to completely load, I'll run up against the flash glitch and the videos won't play for longer than 2 seconds. All of the flickr slide shows appear to count towards the fifteen flash items max, but the flicker slide shows themselves are completely immune to the freezing issues; Only the 11 or so youtube videos freeze. So if you created a test webpage with 21 flickr slide shows, and 1 youtube video, all the slide shows should theoretically play great, and just the youtube videos would lock up. That's my hypothesis. Anyway, good luck with resolving this strange freezing up glitch. I'm gonna use the 15 video max as my "RULE OF THUMB" until this freezing problem gets properly debugged.