Go to Edit (or Tools in case of Windows) --> Preferences --> Security tab.
Under Passwords field, click on "show passwords" and then "show passwords" again. This will show you all the passwords saved by firefox.
eddie is right. If your username and password isn't there, then Firefox hasn't saved it, or it saved it temporarily, for your current session. As soon as you closed Firefox, all the passwords got wiped out. Check your preferences if all your data is wiped out each time you end your session.