I had the exact same problem as CheshireCat. Firefox 3.whatever updated, and then would just keep crashing instantly over and over. THANKFULLY it stayed stable long enough for me to export my bookmarks, I was freaking (I do it weekly because it's a really important asset to me in my work, but I'd just added a bunch on Tues/Wed).
I HAD to download 4.0. I really don't like the look and feel of it, and I LOVE having the status bar at the bottom because I don't always have the fastest (or most stable) internet connection, and I really like to know if a page is still loading or it's gotten hung up. Took a while to figure out there's an add-on called Status-4-Eva or something like that. It's got my Foxclocks and status bar opposite of the way the were (status on left, floxclocks on right), and I'm not happy I can't change it.
It's not that I don't like change... but I was REALLY happy with the way I had everything set up.

Last night I had no problems with 4.0 being stable. But today I keep getting "Not Responding" and it's driving me so nuts, so after reading reviews that compare security of all the browsers, I downloaded Opera 10.0. Now I know what Firefox 4.0 is. It's Opera 10.0.

I'm using Opera right now because it's faster, and I'm already used to navigating the new menu and tabs (even though I'm not happy about it LOL) - but at least it's stable.
I imported my bookmarks (6,015 of them!!!!), so I'm good to go.
Opera does seem noticeably faster - at least with my connection.