The first thing I would try to remove/reinstall the Flash add-on. If that doesn't work I would disable other add-ons and see if that fixes things.
If that still doesn't work, back up your bookmarks and do a fresh install of Firefox with a new account. That should definitely work, but would also involve the most work on your part.
Usually when an add-on has a problem, it is due to another one conflicting with it.
Also, are you using Adobe Flash to play flash in the browser, or something different?