Via twitter I find the news that there is a PPA to install the eighth version of this browser and beloved as Drive_in.com Tech is 20% faster compared to Firefox 5 and also the memory consumption is reduced (even though not specify how much).
To install just write the following in the terminal:
sudo add-apt-repository ppa:ubuntu-mozilla-daily/ppa
sudo apt-get update
sudo apt-get install firefox-trunk