Firefox 4 is already in Beta 8. It's feature complete with only polishing and bug fixing work left to do. So, with the current obsession with speed, what's made it's way into Firefox that will give us the speed we so crave?
Here's a short list.
Hardware acceleration - This is the other feature that gets the credit for Firefox's speed ups. Chrome and Internet Explorer are offloading rendering work to the GPU and Firefox is no different. Not all platforms have this feature enabled yet and as usual Linux users get to wait. Windows and Mac already has it enabled.
Retained layers - Firefox 4 now caches content. Basically, when things need re-rendering things that haven't changed gets reused instead of re-rendered. This helps speed up the browser in some common cases.
That's just some of the improvements that's made it into Firefox 4 to help speed up the browser overall. There's, of course, micro optimisations throughout to help speed things up here and there. All these add up to make a significant improvement to the overall Internet experience.
Firefox 4 Beta 8 released
Installing Firefox 4 pre-Beta 8 on Ubuntu 10.10