I spent a couple days fighting with Firefox 3.6.8 on a fresh install of Snow Leopard. The problem was that add-ons would not install. Faced with life without Adblock Plus and Delicious Bookmarks, I was momentarily driven to Safari, but nevertheless needed Firefox for development.
Mozilla know about this problem, and even have a webpage with a solution: http://support.mozilla.com/en-US/kb/Unexpected+installation+error+-203+when+installing+add-ons. Unfortunately, it didn't work for me. First, I tried removing the supposedly corrupted extensions database files from my profile. Then, I tried creating a brand new profile. But I still had the same error.
It turned out that restarting Firefox was not sufficient. I had to entirely reboot my machine. It took me a day to figure this out, because it's a Mac... I don't reboot it, ever. Well, ok, when there's a major patch and Software Update says reboot. But that's it. If I want an OS that needs to reboot, I know where to find it.
So the exact steps, for me, were:
1. Create a new profile. Do this by running /Applications/Firefox/Contents/MacOS/firefox-bin -P in a terminal window. This starts the profile manager, and you can create a brand new profile and delete your old one.
2. Quit Firefox.
3. Reboot (sigh)
After logging in and starting Firefox, I could finally install extensions again.
