Just a few things I've always experienced when using Firefox (even on different installs of Windows), I'm not sure if others have found the same annoyances...
Clicking on URLs often ends up downloading the webpages to my disk instead of bringing me to the page. I have 33 copies of Coffeehouse.htm in my Downloads folder, and only as little as that because I've deleted some already.
Textboxes auto-complete when I don't want them to, just because I press Enter. Sometimes I've entered a search query with more than one word, and then I want to go back and search with only one word, or sometimes I go to search for something with just less
characters in the query, and when I press Enter, to perform the search, it instead selects the first item from the auto-complete list and does a search I don't want.
Even though there's a dialog for automatic installing for plug-ins, it always makes me do a manual install.