UniquePasswordBuilder Addon by Grégory Paul, Philippe

A good security practice is to use a different and strong passwords for each site you visit.
Memorizing that in your head is a pain.
The UniquePasswordBuilder addon allows you to construct strong and different passwords based from a single master password (the only you have to know).
When you’re on a login page, click on the lock icon, then type your master password. When finished, a password for that login page has been generated. You can then click on the icon on the right or press enter to paste it into clipboard, then just paste it in the login form. For each website, the generated password is different.

The master password is hashed a large number of time with the domain’s site generating an unique password for that site. Hashing algorithm is scrypt (which is memory resistant).

Note that your master password should be strong (more than 10 characters in lower and upper case, numbers and special characters). Using simple password makes brute force attack easier.

Other password tools (LastPass, KeyPass, etc) usually stores websites passwords into a encrypted database.
The drawback of that approach is that the database needs to be synced between all of your devices.
UniquePasswordBuilder doesn’t uses a central database because it recomputes your password each time.
For increased security, you can set a `user salt`, which make it even more resilient against rainbow table attacks.

You can use CTRL + SHIFT + L to open the addon.

That addon is free software and you can study the source code here : https://github.com/paulgreg/UniquePasswordBuilder
You’ll find further information on the project home page : https://paulgreg.me/UniquePasswordBuilder/