Not gonna lie, I was super confused why F-Droid was updating thunderbird beta on my device when I was updating apps a few days ago. Took me a while to realize k9 was apparently changed to thunderbird beta.
According to someone else who replied to my comment, it seems it's some sort of metadata issue rather than an actual rebrand. I'm just an idiot who doesn't actually look deep enough into this kinda stuff.
in any case, if its updating the old app to the new one that is still massively annoying. the new UI is nearly unusable for me since I usually use apps with one hand.