You could start by telling us what the error message was? We don't get to sit in front of your device.

If Thunderbird crashed, reporting the crash id's so someone can have a look is also helpful in seeking a resolution. See https://support.mozilla.org/en-US/kb/mozilla-crash-reporter-tb#w_viewing-crash-reports

I loaded my backup and got the previous state of the disk. Again after the update, Thunderbird does not restart. It does not send any message. Mac OS shows the message "Application not responding". After forcing the application to quit. It starts normally. Now, all the information is loaded correctly.

I got a message (on 91.9.0) telling me to download the new version and that I'd be guided through the installation. I downloaded and installed but nothing has changed and I got no guidance. I have a decade or so of data I daren't lose how do I proceed safely?

I'm running Win 11.

Well I risked shutting Thunderbird down and restarting it and it came up with version 91.9.1 but, a day later, I got the same pop-up which says:-

Thunderbird can't update to the latest version.

Download a fresh copy of Thunderbird and we'll help you to install it.

Learn more

A friend using Linux was updated automatically without all this palaver! Would someone care to explain what is going on please?

Well This guy using Windows 10 had no problems updating either! So I guess you should open your own discussion in your own question instead of hijacking a solved matter.