Do you have the same version of Flash installed on both computers? Adobe just released 11.3, so perhaps one of these machines is different than the other? I was able to load the videos just fine in my version of Firefox (using an older MacBook), so I'm at a loss for what's going on with your iMac.

If you're worried about destroying any settings for Firefox, you can back up your profile using this page

Thanks for the reply!

I always do the updates when prompted, so I'm assuming I have the same version of Flash on both computers. How do I check for the version of Flash..I don't see it in the applications folder.

Maybe I'll try backing up my profile and re-installing Firefox...

In System Preferences, Flash Player is one of the menu items that now exists. Under the Advanced tab, you'll see the version number as well as the option to check for an update

Try to clear Cookies & Cache and check it again.

thank you

FoxBJK, the Flash version is the same on both computers. Ideato, I've cleared the cookies and cache several times with no change to the problem.

Do you think I should just re-install Firefox?

I would, but use a program like AppCleaner so that you not only remove the application, but all the various files stored in other parts of the system. It might not catch everything, so be sure to go into /Library/Application Support/ and ~/Library/Application Support/ and remove any folders named "Mozilla" and/or "Firefox".

see also : Reset Firefox

thanks again