After the release of Ubuntu 11.10, Mozilla has also released Firefox 10 Alpha 1 (Nightly) and is now ready for download from PPA. As you now, this is an unstable version and is still under development. You may receive some bugs while using it.
To install Firefox 10 on Ubuntu 11.10/11.04, open the terminal and run these commands:
sudo add-apt-repository ppa:ubuntu-mozilla-daily/ppa
sudo apt-get update
sudo apt-get install firefox-trunk
You can then start it via the Unity dash: