Another way to lock down Firefox is to set a "Master" password. This is a
special password Firefox asks you to key in once per session. Key in the correct
master password and Firefox works just like it used to work by auto-filling your
saved usernames and passwords on your favorite sign-in pages. Key in an
incorrect master password, however, and Firefox automatically blocks your saved
usernames and passwords from displaying. Sign in pages will still load, but the
username and password boxes will be blank. 
 
To set a master password:
1. Go to Tools > Options > Privacy
2. Click on the + sign next to the words "Saved Passwords" or, in newer versions of Firefox, click on the "Passwords" tab.
3. Click on the "Set Master Password" button.
4. Key in a new "master" password.
5. Click on OK.