Firefox Help Please! Automatically Decompress Files?

I just started using firefox instead of safari when they released firefox 3. Everything has been great and I really like it, but I do have on problem. When I download a dmg file, unlike safari, firefox does not automatically decompress it and pops up a dialog box, which rather annoys me. I was wondering if there was any add-on or preference I could use to make it not pop up the dialog box and automatically download and decompress the file. Any help would be much appreciated. Thank You, Jesse