It’s actually the opposite, TBirdMobile is a fork of K9 (and it’s basically just reskinned blue for now lol), they currently have feature parity but at some point (supposedly already, but I guess they delayed it because K9 got an update a couple months ago) K9 is set to stop receiving updates as they fully switch over to TBird.

I also love K9, been using it since ~2012.

At least for now, it is just blue K9, but so help me if Mozilla hurts my sweet app I’m gonna throw a fit lol.