Counter-anecdote: I use Firefox all day every day and usually have 100s of tabs open. I haven't had a legitimate full-browser crash in years. I've had a hang that resulted in me having to kill the process probably 3 times in the last year. I usually don't enable Flash, but when I do, it usually doesn't crash, and when it does, it just takes out all instances of Flash currently running; doesn't bring down the whole browser. I'm on Linux.
If you're getting a lot of Flash-induced crashing, you may want to check into alternate implementations of Flash, like Gnash, Lightspark, or even Mozilla's Shumway.
It shouldn't. At least on Windows I've gotten used to killing the plugin container about once ever 2 days.
Firefox itself crashes once per month, maybe less.
Firefox = rock solid. Flash = porous rock, at best :)
You should go to about:support and use Refresh Firefox which will probably fix most of your crashes (outside of Flash-related ones which should only be affecting the plugin and not the browser).