I also daily use Firefox, Thunderbird, VSCodium, Signal, KeepassXC, DAVx5 and Radicale for calendar syncronization.
What I don’t use each and every day but what are among my favorite FOSS projects are KiCad for electronics design and CoMaps for navigation.
You make a copy of the code (“fork”) for yourself, make edits, then request that your changes be accepted into the original project (“pull/merge request”). Someone from the project has to check the edits, make that decision and hit accept or decline.