Ever since Adobe Flash updated to 11.3, it had been unstable for me and crashed too often. I tried to uninstall and reinstall it, but it still happened. I even waited until Firefox updated again, which it did a few days ago, to 14.0.1. It still crashed.
So a couple of days ago, I uninstalled Flash 11.3 and downgraded to the latest release of Flash 10.3 with all the needed security patches.
Uninstalled 11.3.300.265
Installed 10.3.183.20
So far I haven't had a crash or a problem with Flash in Firefox since.
If you want to downgrade to 10.3, check out this page on how to do it.
http://support.mozilla.org/en-US/kb/flash-113-crashes