firefox-nightly 20.0
Nightly introduces Firefox 20.0. You can see what's planned for this version by visiting the Fixtures/Release Tracking page, although note that most of these features are not implemented in Nightly (such as Windows 8 Metro support). One interesting point to note is that Nightly also offers a 32-bit version of Nightly for Windows as well as Linux.
firefox-20.0a1.en-US.win32.installer.exe