SmoothWheel - An extension that has a smoother scroll for FF.
enhance Google search results and remove ads and spam
SearchEngineOrdering - Basically let's you change the order of the engines in the top right search bar on Firefox. It also lets you delete engines by right-clicking and deleting them as opposed to going into the folders and stuff.
G-Mail Preview Bubbles - Self-Explanatory
GreaseMonkey - Let's you use "user scripts"' for any web site
Blue Security - Anti-Spam for e-mail.
AdBlock Filterset - Blocks popular ads autmoatically.