I wouldn't trust it to keep your bookmarks, but you can save them - should be in Documents & Settings/Application Data/Mozilla/Firefox/blah blah
You'll find a bookmark.html in there...just open it up and compare it to your actual bookmarks just to make sure it's the right one.
I didn't have any problems though, just downloaded and installed the new version - and it didn't delete my bookmarks like when I updated from 0.8 to 0.9. I didn't remove the old version before installing the new.