So instead of just waiting until you restart firefox, it forces you to restart it RIGHT NOW, usually in the form of showing a bunch of "GAH! This tab just crashed!" pages that crash over and over if you click the retry button.

Only when you try to open a new link in a new tab does it tell you what's really going on (with an annoying cutesy "We just need to do one more thing" - Well FU do it later, I'm in the middle of something!). Doesn't matter that you'll lose work if you restart now - you're just screwed. It's like Microsoft forced update, only much, much worse.

And then once it restarts and you try to pick up the pieces, you discover that it loses the last tab you had open, so you have to remember what it was and then use a search engine to find the site again.