The glitch between Adobe Reader 6.0 and Firefox is pretty well-known. Not only is Adobe Reader 6.0 a dog (on any platform), but in Firefox it can hang the browser when you try to exit the PDF file. Annoying.
One of the standard solutions is to upgrade to Adobe Reader 6.0.1. This seems to clear things up.
Problem is, I don’t use Adobe Reader. I have an actual copy of Adobe Acrobat 6.0 Standard (to write PDFs), and despite efforts to do the “upgrade my version” stuff in it, it didn’t want to advance beyond 6.0.
For the record, and in case anyone else is facing that particular dilemma, doing the Reader upgrade to 6.0.1 seems to work. You end up with multiple icons, but so far both the Reader and Acrobat seem to working find, and windows/tabs can be closed without hanging Firefox. Shiny!
I also downloaded a program called Adobe Reader Speed-Up (ARSU), which disables a whole slew of the Adobe plug-ins, which in turn speeds up the Reader’s load tremendously. (The newest version makes the plug-ins optionally loadable by PDF, so you’re not really crippling the program.) I recommend it.