Use Gmail? Use Firefox?

Of course you do...

Try this firefox extension, it's an excellent modifier that give you plenty of funky options to improve your Gmail experience. Essentially, it's a bunch of Greasemonkey scripts (http://en.wikipedia.org/wiki/Greasemonkey) put together into one big extension.

Link to Lifehacker (Better Gmail)

All I would stress is that you may want to limit the options you choose as it can slow down Gmail considerably if you get too greedy.