Iceraven is a fork of Firefox mobile with about:config support, among other things

Unfortunately it's not currently on F-droid but it seems like that is a planned goal.

Edit: I came across this project while looking for an alternative, because Fennec F-Droid did not sync my history with my desktop browser when I installed it. I did, however, reinstall it and this time it worked, so I am now using Fennec.