I was looking for an open source password manager and found Pass. Pass uses gpg to store passwords in encrypted files and also offers integration into Firefox and other browsers.
How to setup
This setup is suited for Mac OS and assumes you start from scratch. All activities are to be performed in the terminal unless otherwise specified.
Before you can use pass, you need to setup gpg. Here is a short introduction on how to setup gpg: http://wooledge.org/~greg/crypto/node41.html
Don’t forget to make a backup of your key in case it gets lost: https://www.jwillikers.com/backup-and-restore-a-gpg-key
Setup of pass: https://www.passwordstore.org/
Setup of pass-host to allow communication from the Firefox plugin to the pass-store: https://github.com/passff/passff-host
Integration into Firefox https://addons.mozilla.org/de/firefox/addon/passff/ Further Details: https://github.com/passff/passff
Install Pinentry-Mac if you’re on a Mac to enter your password: https://github.com/passff/passff/issues/325
How to use
Before you can use pass, you need to initialise it. You can use pass together with git to version your password-repository. Here is a step by step guide how to setup pass with git: https://git.zx2c4.com/password-store/about/#EXTENDED%20GIT%20EXAMPLE
Afterwards you can populate your repository by storing credentials into the password-repository in a special format. Details on the format can be found here https://github.com/passff/passff#readme in section Password formats.
Then in Firefox you should see the passff-extension icon in username/password fields. On click, you can choose from your credentials to fill in.