The salt is hashed using HMAC keyed with the hashed password - this salt is then used, alongside with the provided master password, to generate the final key using bcrypt, an algorithm designed to be computationally expensive.
The keyed HMAC expansion for the salt prevents rainbow tables generated for a fixed salt - this way the salt provided to the derivation function(bcrypt) is also a function of the key.
Press 'F9' to get the toolbar to show - pressing F9 again, the toolbar will hide. Just type in your master password, a salt for a given page (e.g. "amazon" or "gmail"), click "derive key" and the derived key will be shown and also copied into the clipboard.
If you need to access your passwords but can't use firefox, there is also a chrome plugin under the same name and a web version (which runs purely in your browser, nothing is ever sent to the server) at http://u.alexhornung.com/bpasswd
For a more up-to-date version for newer Firefox versions, see BPasswd2