Here is a guide to install the daily builds (development versions) of Firefox and Thunderbird. You can install these versions using the Ubuntu Mozilla Daily Build Team PPA for Firefox and Thunderbird.
To add the PPA open a terminal window (press Ctrl+Alt+T) and copy+paste the following line:
sudo add-apt-repository ppa:ubuntu-mozilla-daily/ppa
To install the Firefox daily build copy+paste the following line:
sudo apt-get install firefox-trunk
When the Firefox daily build is installed you can close the terminal window and to start the Firefox daily build press the SuperKey and type: nightly web browser
To install the Thunderbird daily build copy+paste the following line:
sudo apt-get install thunderbird-trunk
When the Thunderbird daily build is installed you can close the terminal window and to start the Thunderbird daily build press the SuperKey and type: daily mail
Suggested packages: thunderbird-trunk-gnome-support latex-xft-fonts
sudo apt-get install thunderbird-trunk-gnome-support
Please not that the latex-xft-fonts package is no longer available.