Recent crashes of certain multimedia contents (this includes Youtube videos, certain flash games and other applications) in conjunction with Firefox are most probably caused by a recent Flash update and/or a malfunctioning browser plugin such as Real Player.
In order to remedy the problem, please perform the steps mentioned in these Knowledge Base articles:
This is about Flash on Linux, so protected mode and other Windows issues do not apply.
The last version for Flash is 11.2.x and Adobe only provides security updates for this version.
Google Chrome uses different pepper Flash version on Linux that is still being updated just like the Windows version.