All Firefox passwords are stored in three files
signons.txt, signons2.txt, key3.db
These files are stored in your Firefox profile folder.
In Windows XP Firefox profile is at:
c:\Documents and Settings\Windows Username\Application Data\Mozilla\Firefox\Profiles\profile name
Copy these three files and save them where ever you want and if you want these passwords in your other os also then just replace these three files.
Install export Firefox passwords add-on and import it in another Firefox…. simple