Be sure to read the "Introduction about Password Composer":http://www.xs4all.nl/~jlpoutre/BoT/Javascript/PasswordComposer/
and the "FAQ":http://www.xs4all.nl/~jlpoutre/BoT/Javascript/PasswordComposer/faq.html before using this user script.

From the FAQ:
The Password Composer gives users the possibility to remember one single password for multiple web sites. In reality, every site gets its own, generated password, transparent to the end user. This means that you only need to remember a single, strong password. Still, a compromised password on one site does not mean they get access to any of your other registered web personalities.