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.