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.

That's weird, since Flash runs on its own process (plugin-container). Is it actually crashing the whole browser?

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).