BadPass is a Firefox add-on designed to promote stronger passwords through the use of a password strength indicator that is applied to all HTML password input fields; a database of over 500 common and easily guessed passwords; and the option to enforce password constraints before submitting any HTML form.

The BadPass password strength indicator working on the twitter.com website
By default BadPass requires any password you enter in a HTML form to be at least 8 characters long and contain lower and upper case letters, numbers, and symbols. It also requires that the password does not exist within the included list of over 500 commonly used passwords. The strength of a password is indicated as you type by changing the background colour of the password field. All of this can be configured to suit your needs.

The notification displayed when a bad password is used in a form
Click here to install BadPass 1.2.
After installing BadPass you can access the preferences dialog through the Tools>BadPass Preferences menu. From here you can configure the BadPass settings and the colours used by the password strength indicator. You can also view and modify the list of common passwords.
Please note BadPass does not store or share your passwords with anyone, it simply performs text comparisons.
If you experience any difficulties while using BadPass or have any suggestions to make please feel free to leave a comment or contact me and help me to improve BadPass.