See also https://forum.endeavouros.com/t/floorp-going-closed-source/52783
Edit: They claim they will make that part open source too, eventually, and it is due to behavior of another browser: https://github.com/Floorp-Projects/Floorp-core/issues/62
Edit 2: They just open sourced the private repository 7 minutes ago, 2024-03-24T12:39Z
Even LibreWolf, which is just a Firefox with different configuration by default, I think should be just config files.
I really want to see browsers saving configurations and data in a simple file formats, like QuteBrowser do.
You may be looking for Betterfox.
In Firefox, you can use the user.js file to create any overrides you want.