You likely either have a Flash issue (it can cause problems sometimes) or you simply have a corrupt profile.
1. Try re-installing Flash. It's pretty easy to do.
2. Try removing your Firefox profile so that you can create a new one.....
First, locate your existing profile:
http://support.mozilla.com/en-US/kb/Profiles
Within your Firefox profile folder, you'll find a file called Bookmarks.html
Save that somewhere because it is your list of bookmarks.
Once you've done that, you can either just delete the entire profile directory (*.default) or you can launche the Firefox profile manager and delete it from there and create a new one.
To launch Firefox's profile manager, go to your Start menu, go to Run and enter
firefox -profilemanager