If you have a username and password that has changed, but Firefox keeps trying to automatically fill in old passwords for you? Delete them with these steps.

1. In Firefox click Tools > Options.

2. Click Security at the top.

3. Click the Show Passwords button.

4. Find the web site you wish to remove in the list. Click/highlight it with your mouse.

5. Click Remove and click Close then OK when you are finished.

Now the stored password will be removed from Firefox.