Your saved usernames and passwords are stored in the file signons.txt (in an encrypted form, not plain text) in the profile directory (local drive:\Documents and Settings\username\Application Data\Mozilla\Firefox\Profiles\random characters.default). Firefox does not back this file up automatically, so unless you backed up your profile previously (you can restore that one file), your only option is to re-enter them one by one.
Can I retreve saved passwords that I accidently deleted from Mozilla Firefox 1.0.7?
I accidently went to delete one password and clicked on the wrong button. I removed all my passwords. If there is a way to get these back, it would be easier than entering them all again one by one.