Yesterday I downloaded the Better Gmail Firefox Extension from Lifehacker and I have to say, it's sweet.
So far, my favorite features are the additional Keyboard Shortcuts. Before I used the built in shortcuts for next j, previous k, select x, and compose c.
Now, I can label things with l, trash things with t, and read/archive with d.
I haven't delved much into the other features like "Saved Searches" yet, but this extension is easily on track to becoming my favorite (after firebug).