This was my experience while using Firefox with flash on OSX for as long as I can remember. I had the same problem with safari, but I haven't use safari on mac for a long time, so I can't verify that. I only use Chrome now and before the recent update where they integrated flash beta player into Chrome[1], Chrome also used to crash very frequently. Even apple revealed that "based on data from the Crash Reporter application built into Mac OS X — the thing that asks if you’d like to send crash data to Apple after a crash — the most frequent cause of crashes across all of Mac OS X are “plugins”" [2].

[1] http://blog.chromium.org/2010/03/bringing-improved-support-f... [2] http://daringfireball.net/2010/01/apple_adobe_flash