If you download the appropriate package then all you have to do is extract the archive in GNU/Linux and run the executable Firefox 5 file which should let you run the new browser. But, this unlike with the usual method of installing it through your own software manager, will not create any icons nor menus, etc... so it kinda sucks.
In that case you'll be glad to know that, the Firefox 5 is actually in the Ubuntu Linux Testing channels. This is actually a standard procedure used (well, not just Ubuntu developers) by Ubuntu OS developers, before they made the official release of the any software it first "hits" the testing channels which anyone can participate by installing and using then giving them the feedback, etc.
Anyhow, as mentioned before, if you can't wait few more hours (actually it can be more than few hours, since we don't know when Canonical will make it available via Launchpad) until it's officially released, then you can install the testing version of Mozilla Firefox 5 in Ubuntu 11.04 by using the below PPA channel.
sudo add-apt-repository ppa:ubuntu-mozilla-security/ppaRemember, this will only work for Ubuntu 11.04 version only. And as said before, this is for testing purposes only and if you're concerned about security and want the most secured version of the Firefox web browser then wait for few more hours (I'll update you as soon as it's available for GNU/Linux). :D.
sudo apt-get update
sudo apt-get dist-upgrade
Oh one more thing, it seems like the only available language pack is English.