Update: Firefox 5 is now available in the official Ubuntu 11.04 security repository so you no longer have to follow the instructions below to upgrade to Firefox 5. Just run a normal update.
Firefox 5 is available to download via Mozilla's FTP servers and because this is a security update for Firefox 4 (which will no longer get updates), it will be available in the main Ubuntu 11.04 repositories soon (I'm not sure when though).
For now, Firefox 5 is available in the Natty proposed repositories, for testing purposes only. Once it's properly tested, it will be uploaded in the main repository. If you want to test Firefox 5 (final), enable the proposed repository in Software Sources (in Ubuntu Software Center, click Edit > Software Sources and enable the proposed repository) and you should get a Firefox update. But please note that enabling this repository, you'll get a lot of other updates which are currently in testing (I'm using it and everything seems to work ok, but it's not recommended you enable it for anything other than testing purposes).
Firefox 5 is also available in the Mozilla Security PPA (for Ubuntu 11.04), but if you use Firefox from this PPA, anything that requires a language pack will fail. If you want to add the PPA anyway and help test Firefox 5, use the commands below:
sudo add-apt-repository ppa:ubuntu-mozilla-security/ppa sudo apt-get update sudo apt-get dist-upgrade
Firefox 5 bugs for Ubuntu 11.04 are tracked here | Read the "Call For Testing: Natty Firefox users" announcement