This is not something I experience at all. So I am looking for things that might be different for you and me.

The first thing I see is you are using Linux. So the next obvious question is what flavor of Linux and where did you get your Thunderbird. Unfortunately unlike Windows or Apple operating systems no two Linux distributions can be considered the same. Linux also has many versions of Thunderbird and they are not all the same. Maintainers build their own in some circumstances. Others use flatpak. Like everything Linux nothing is clear cut or simple

I didn't experience it until I updated the OS to Debian 12 - which came with a new version of Thunderbird (package version 1:115.8.0-1~deb12u1). I thought of checking the maintainer's compilation options but it didn't seem likely to be a configurable compile option. I do have a sizeable user.js (custom config) but sometimes finding the obvious meaning and/or consequence of modifying a setting is unclear. I thought about running through each 'could be causing the issue' setting in the user.js file but that can be very time consuming (and ultimately fruitless). I may end of doing that anyway if it's not obvious to someone reading these posts why Thunderbird is creating this tab upon every startup. Thanks for responding. Much appreciated.