After the release of Firefox 8.0 as stable version, all other channels of Firefox will be updated soon. The Nightly channel of Firefox has got Firefox 11 (Firefox 11.0a1), and the Firefox 10 will be upgraded to the Aurora channel soon. Similarly, Firefox 9 will go the beta channel for testing the upcoming features.
To download Firefox 11 Nightly build (32 bit & 64 bit) for Windows, Linux, Mac and Android platforms, head over to Mozilla FTP here and choose the latest release for your platform. Just to remind you, nightly builds are only for preview and development tasks, and may contains some bugs and incomplete features.
Expected release date of Firefox 11 is 13th March 2012.