it probably is one of your Firefox Add ons. In Firefox click on the Tools menu (or if you do not have the menu bar click the Firefox dropdown in the upper left corner) and select to manage your addons. If you can't spot the problem addon simply DISABLE all you are not sure about then try the video clips. If you cn now see them re-enable the addons ONE AT A TIME and try accessing the videos until you find the one causing the problem.
Once you identify it you can decide to either leave it alone if it is an add on you depend on or to remove it.
I have a odd problem. I cannot go to the cnet site (cnet tv) and view their video clips in firefox. I can view Youtube. Dailymotion, metcafe, etc. But the only way I can see cnet is if I switch firefox over to the IE tab. I also have other general viewing glitches in firefox with cnet. For example. there is this little tab at the bottom of the cnet screen that i cannot "X" out. I can do it in IE but not while I am using firefox. I must have some kind of ad blocker software that keeps the page from loading correctly. Or maybe it is something else. Any ideas??