Windows: If you'd like your web browser to run a bit faster, Waterfox is a Firefox clone built to run faster on 64-bit processors.
Mozilla still hasn't officially released any 64-bit builds for Firefox, and Waterfox aims to fill that void. It has one main goal: speed. The developers have tweaked a lot of Firefox's codebase to work better with 64-bit processors, so if you're running a 64-bit version of Windows, it should give you a solid speed boost. It also updates whenever Firefox does, so you're never behind, and uses your current Firefox profile, so you don't have to do any extra setup. All of your add-ons should work out of the box, too.
If you don't like Waterfox for any reason, you might also check out previously mentioned Pale Moon, another Windows-optimised Firefox build. It updates a bit less quickly than Waterfox, and lots of users find Waterfox to be faster, but Pale Moon also has a few other UI tweaks. Hit the link to check out Waterfox.
Waterfox is a free download for Windows only.