DataDrake
I found the official instruction from Thunderbird, in case anyone's interested. When they say to download it to the same location where you installed Thunderbird 91, I suspect they're talking about a manual installation from their web site, not an installation from our Solus repository.
It's good to know that -- eventually -- version 102 will simply replace version 91. So apparently we'll automatically get updated when the time comes.
I'm going to leave my old version 91 where Solus put it, and let it be updated when the time comes. That should preserve my profile and everything in it, including all the message filters I've created over time. And take care of its entry in the Start menu. Then, I'll remove the version that I installed in the new folder I created in my home directory.
So, until a Solus update takes care of my original Thunderbird, I'll use the new one I installed manually, and after the original one has been updated, I'll remove the one I installed manually.