I wrote a long message in a web form in Firefox. Then in another tab, I did something which caused Firefox to hang (opened a 20 MB binary file which displayed as text because I had the wrong MIME type server side). I had to force-quit and restart Firefox.
I was delighted and surprised to see that Firefox remembered the partially completed form on restart. While eradication of hangs and crashes should be the top priority in any application, naturally, this kind of recoverability contributes a lot towards user confidence.