I have the same problem with firefox on one of my slackware machines. I think it is somehow related to Flash Player. If I open a Flash intense website, such as
www.cigarsinternational.com, and leave it opened, but minimized, the other, subsequent instances of firefox don't close suddenly. If I close the Flashed up website, then any other instance of firefox will suddenly close soon after.
I've been meaning to look into this and try installing a different version of flash, but it's such a pain. I'm eventually going to upgrade that machine to slackware 12 and hope the problem just goes away.