Uninstall and reinstall flash. Maybe it got messed up at some point? It may not hurt to reinstall FF as well(back up your bookmarks first.).
Does anything else act strange on your system? Does flash work right under IE? .This could be a sign of other problems.
I have been having a similar issue with flash, system wide. I have issues with it crashing in chrome, firefox, and IE. i run a windows 7 64bit system. tried all the standard trouble shooting including fresh reinstalls of flash and all of the browsers. this is affecting a great deal of my browsing.
is it possible to have a adobe flash related virus? or malware? ive run two separate, fully updated antio virus programs and no luck figuring it out.