FireFox 30 and Flash Player 14 are current and have fixes and improvements. I will not guarantee it will fix your problem, but have you tried updating? It appears FireFox 29 and Flash Player 13 were a bad combo for some people. I had lots of FF (or was it Flash) crashes. I just recently updated to FireFox 30 and Flash Player 14. It's early, but things seem to be OK so far.
I did have 'choppy' Flash the longer I ran FF (especially with lots of Tabs open - which I regularly do) before either a reset or a crash. I'll see how it goes with the latest updates I did.
I recently had to delete my YouTube Cookies to stop the WebM/HTML5 and get back Flash playback.
- Pj