Firefox protects its stored passwords with a "Master Password" which is not stored anywhere on the system but used as an encryption key. That means the passwords are scrambled with the Master Password using a very strong algorithm and the only practical way to unscramble them is by entering the Master Password. If you have lost the Master Password, it does not exist anywhere on your computer and therefore cannot be recovered - and that means that the site passwords are also lost.
However, if you have not forgotten your Master Password, or if you do not use one at all (which is the case for the huge majority of Firefox users), to see the site passwords! Simply go to Tools :: Options :: Passwords in Firefox and select "View Saved Passwords". If the passwords are not already showing after that, simply click "Show Passwords" and they will be visible.