For Android 2.0 upwards and Maemo devices. Large download, perhaps only for those who can instal apps to SD card:
"Our first Firefox 4 beta for mobile is now available to download and test. It’s built on the same technology platform as Firefox for the desktop and optimized for browsing on a mobile phone. Firefox beta for mobile comes with many of your favorite Firefox desktop features like Firefox Sync, Add-ons and the Awesome Bar."
Page linked to includes a further description, Links for release notes, FAQ and support.