Method 1:
If you have access to the windows account where the passwords are saved then the solution can be simple. Open Firefox and go to tools>options>security>show passwords. This will list all of the accounts with saved passwords. If you click the show passwords button it will display all of the correlating passwords.
Method 2:
This method is a little more advanced, but can be used to recover the passwords of any account on the machine. The passwords are saved in the directory \Documents and Settings\
Attack Mitigation:
Use a master password in your Firefox browser. Go to Firefox>tools>options>security and check the use master password box. This will cause Firefox to ask for a master password before displaying the saved passwords. There are some software packages capable of recovering the passwords even with the master password set, but this will stop casual hackers.