I encountered the same problem with firefox 13 but I did not realized it was
flash-related.
Just to let you know I opened a bug at firefox:
https://bugzilla.mozilla.org/show_bug.cgi?id=762172
and put a link to this discussion in it.
The symptoms I see are exactly as you mention here and all this happened after
the last firefox update. I also updated the flash plugin with no success.

The application we develop here requires the flash plugin so I had to switch to
chromium browser until this issue is solved, no choice :-(

I will post again if I find a workaround or a solution to this issue.

I use Trinity with Ubuntu 10.04 64 bits and it is a charm!