There should be a simple solution to this...

I'm trying to get Firefox to bring up the DUN auto-disconnect prompt when closed. For the life of me, I can't seem to find a good solution. I'm installing Firefox on my dad's machine and if it doesn't work seemlessly, he'll just start using IE and fucking Windows up again.

any suggestions?

Firefox has its own discussion forum on their own site. That forum is heavily used by Firefox gurus, and so you might get better answers there.