This has to be my favorite feature (I don't really use FireFox, but just started to play around with it). I am always looking for specific text in a web page, and I hate doing the Control-F, rinse, repeat mechanism to find something. FireFox has this nice feature where you can just start typing the text and it will automatically find it in the page.
I do this A LOT when clicking on results from a search query, and looking for the string I searched for in the page. Love it.
FireFox is pretty slick. It has lots of nice small usability and productivity enhancements.