Yep, if you don't want to use Sync, user.js is an alternative. It's also helpful for sharing settings across multiple profiles. (For the latter, I do wish there was support for including other files...)

There is a user.js project on github[1] maintained by ghacks.net community members that provides a source of documentation for many settings. It's also useful as a way to monitor new and deprecated settings.

[1]: https://github.com/ghacksuserjs/ghacks-user.js

thanks for this!! Hope this will solve my main annoyance with Firefox: sometimes I change Linux distro and then I lose all of my firefox settings, ("sync" would be better and keep flags changed by the users, but I don't think that the FF devs bother about it)

Uh didn't know that, I'll have to redo my `about:config` tweaks in that format.