I’ve had this problem ever since Safari 4.0 came out a little while ago. The symptoms are simple, when you close a Safari window it crashes. I reported it the first couple of times to Apple using crash reporter, then I started to look around for a solution, but to no avail. I gave up for a while and used firefox again. Today I got fed up with this and started to debug the issue from command line and finally found the problem.
A little while back I had purchased a QNAP NAS device, which I absolutely love. At the time I was impressed by the fact that it supported both XP and OSX. There is a piece of software you install on your machine called QGET which allows you to pass all sorts of downloading commands to the NAS (so it can download things in the background). This program has a Safari plugin that turned out to be the culprit. The QGET program is actually fine and by itself doesn’t cause any issues. All you need to do is delete QGET plugin folder from /Library/InputManagers and restart your machine…….No more crashes…..yeaaaaa :-)