What you need to do is find your Firefox profile folder.
Vista C:\Users\<username>\AppData\Roaming \Mozilla\Firefox\Profiles\<profile name.default>
XP: C:\Documents and Settings\<username>\Application Data\Mozilla\Firefox\Profiles\<prof ile name.default>
Within this folder there are some files called "signon.txt", "signon2.txt" etc. Now there is also a file called "signons.sqlite".
Firstly, make a backup of the profile directory. Then copy all the signon files across and start Firefox. With any luck all your passwords will be available.
Now I've never done this myself, I have always used the password addon to backup and copy my passwords so your mileage may vary.
Good luck and hope it helps.