Just a note that this happened to me, maybe this isn't the correct place to talk about it, but I found no other... I keep my browser (of course) and many other programs on my 4GB cruzer, I often times have family and friends who have issues with their own computers and I therefore carry many adware/spyware detection programs and whatever the best free antivirus program I can find on it. Well I recently wiped McAfee off my Mom's computer and installed AVG antivirus in it's place, and at some point in all that I was running and fixing, I ran my portable firefox. Well apparently this allowed the AVG to install it's browser bar and low and behold, the next time I went to run portable firefox, my theme went to default, and all of my add-ons were non-operational. It wasn't until I found the AVG bar items at the top of my list of add-ons that I realized what was going on. I shut the 2 items down, and everything was back to normal. The odd part is, I never told it to install onto my Firefox. My mom's computer uses IE (ugh, she's 67 years old, I cant make her change browsers now!).
DocSalt, you don't say whether you had your Cruzer plugged in when you installed AVG, but their new version 8.0 has a built-in LinkScanner security component that attaches itself to both Internet Explorer and Mozilla Firefox automatically upon installation by default.
If you didn't have the Cruzer plugged in, AVG must have detected the browser coming online as soon as you inserted it and automatically added the LinkScanner to the portable browser, perhaps thinking that it was a new browser installation.
Sounds like you deactivated the toolbar and the LinkScanner, but they are still part of AVG. If you need to learn more about that AVG 8.0 feature, read LinkScanner and their FAQs.
I have AVG 7.5 installed in my PC but haven't ugraded to version 8.0 because of reported problems with their LinkScanner feature. Maybe someone else will respond to your post and say if the same thing has happened to them. Thanks for the heads-up!