I found this interesting article on the Lifehack website. The articles lists 15 tweaks and tips for Firefox, some generic and known to many, others not so much. Did you for instance know that you can move a page down by pressing the space bar, that you can move around tabs by pressing CTRL + TAB or that you can use so called smart keywords for searching IMDB in the Address Bar (try typing in IMDB Johnny Depp) ?
I especially like the keyboard, mouse and tab shortcuts which listed several shortcuts that I have not been using actively but will do so from now on. Several tips in the list however are more or less known by everyone who read at least one "How to tweak Firefox" articles.
Update: Here are all 15 tips to give you a better idea whether it is worth visiting the site for an in depth walk through of them.
Ghacks is a technology news blog that was founded in 2005 by Martin Brinkmann. It has since then become one of the most popular tech news sites on the Internet with five authors and regular contributions from freelance writers.