According to a recent message from Thunderbird engineering director Andrei Hajdukewycz, the open-source mail client will not extend support to older PC platforms as Firefox has done. Mozilla recently announced that the latest “Extended Support Release” of its web browser (Firefox 115 ESR) will receive an additional six months of security updates for Windows 7 and 8. However, the Thunderbird team has decided to take a different approach.
Mozilla Thunderbird run by the Mozilla Foundation
No it isn’t
I believe it is its own thing