This is an excellent website, it basically lists all the changes in the nightly Firefox builds.
If you like living your internet life on the edge then you should try some nightly builds, I do and I'm seeing the future of Firefox right now.
Some of this stuff is running Cairo, which is the OpenGL interface to Firefox, it's as if Longhorn was here already.
Be careful though, some bugs introduced in these builds can be very dangerous if you don't know what you're doing.
Check out the URL to see the 30-40 new features and fixes that happen every night.
http://www.squarefree.com/burningedge/
IE7?
As far as I'm concerned you can stuff it.