Well, I think I see what's going on... "Version 78.13.0, first offered to channel users on August 10, 2021", which makes all the sense in the world to me.

Then, "Version 91.0, first offered to channel users on August 11, 2021"... and, "Thunderbird version 91.0 is only offered as direct download from thunderbird.net and not as an upgrade from Thunderbird version 78 or earlier."

However, I'm not sure why Thunderbird would offer me the v91 update, but not install it.

I would expect it to offer the 78.13 update and install it. While I have been out of the loop for a few days, the normal process is to offer update to the new version on release of the XX.1 release. That allows the direct downloads to identify any huge issues with the actual release before everyone starts upgrading which contains it's own set of potential pit falls.

But was the update offered by Thunderbird? With the release to V78 which was delayed by some 2 months, it became apparent that assorted security software, various update products and a collection of other random sources were offering updates to users despite the Thunderbird update process being effectively turned off with the update rate set to Zero. The situation arose because the Enigmail changes were not yet ready for those that had previously used the addon, but worked for new installs/ PGP users. But we did learn in the process that there is lots of shoddy software offering to find and even install your addons for you. Much to the unset of the enigmail users who trusted their software to update for them..

Interestingly, I checked the browser version just now and Firefox is telling me that version 91 is installed. Yesterday, knowing that v91 was available for download, I check the version of my Firefox and it offered the v78 update. I accepted the update and restarted the browser. Usually when I do that, I get a dialog that says "Firefox is installing update", or something to that effect. But, that didn't happen. So, I wasn't sure if the update installed or not, and when I checked, it still indicated v78. But, today, it now indicates v91. So, I don't know what happened, but, the end result is, Firefox is updated. All is well in the flaming foxbird nest.