Here's an interesting problem for you...

Using Firefox 3.5 browse to a site that displays a ton of Javascript pop-up dialogs (or a buggy site that accidentally does that.)

Kill Firefox.

Reopen Firefox. Guess what - it's going to display the same crap again because it will automatically restore the session without asking confirmation.

If this happens to you, you can always delete the contents of the Cache directory in your profile, but another preventative method I found online is to set an option in Firefox.

In the address bar, enter "about:config". You'll get a warning about it voiding your non-existent warranty. After clicking "I'll be careful", right click anywhere in the list, pick "New - Integer" and name it:

browser.sessionstore.max_resumed_crashes

Set the value to 0 (zero). This prevents Firefox from trying to resume the session after a crash. Perhaps you want that value higher, but for me, if my browser crashes it's almost always caused by the page I'm viewing.