Check it out (excuse the colors)
. There's a small chance it will be in Mozilla 1.8 beta1, a larger chance it will be in 1.8 beta2, and a very good chance it will be in 1.9 alpha. It's currently also targeted for Firefox 2.0.
And when this gets added, you won't need an extension.I'm running Moz 1.7.5 with the Multizilla plugin and already have that ability, plus quite a bit more...