Export your Firefox 3.0 full profile to Firefox 3.1 (including all addons, themes, cookies, passwords, etc)

This article explains how to move a full profile (addons, themes, cookies, browsing history, passwords and so on) from Firefox 3.0.x to Firefox 3.1.x beta or any other version, but it also works for synchronizing Firefox on 2 different computers or backing up a full profile of Firefox.