Same thing happens when I use another macbook running on 10.6. It only started when I updated to Firefox 16. It was also the day that I started using the extension Siphon. So I disabled Siphon but it still continued till today.

So right now whenever I want to restart my firefox I have to check the add-ons tab if any extensions were removed before I could quit it. Last week I didn't check, so when I restarted, 17 of them were removed. Particularly one of them was the Bookmark Favicon Changer, and that's where I have another question.

Usually when I remove the add-on and install it back, my favicons would return to the existing bookmarks on my toolbar. However this time when that extension was removed, all the favicons which I modified weren't there anymore. I have no idea how to restore them.