Is your computer managed by your employer, school, or another organization?

Check the update debug log and other update files:

1. Settings > General > Config Editor (https://support.mozilla.org/en-US/kb/config-editor)
2. Paste in app.update.log and set it to true
3. Open the Console either with the hotkey Control+Shift+J (Command+Shift+J on macOS), or via Hamburger Menu->Tools->Developer Tools->Console
4. Near the top, in the text field "Filter Output", enter AUS: to select just messages related to updates. Check that Logs is highlighted at "Errors Warnings Logs".
5. In Settings' search field type or paste "updates", then click on the button "Update to xxx" or "Check for Updates"
6. Once the update check has completed, copy the messages out of the Console and attach them here.
7. In same area of Settings click the button "Show Update History" and screen shot it
8. If the update process began and is being investigated, do Help > Troubleshooting Information to find "Update Folder", click "Open Folder" or "Show in Finder" and expand that directory to find last-update.log, last-update.log and backup-update.log and attach

First off, thank you very much for your quick (and clear) response.

The computer is mine - I manage it completely.

I did as you suggested and this is the output:

34 hidden AUS:SVC Checker: checkForUpdates, force: true AUS:SVC UpdateService.canUsuallyCheckForUpdates - able to check for updates AUS:SVC UpdateService.canCheckForUpdates - able to check for updates AUS:SVC Checker:getUpdateURL - update URL: https://aus.thunderbird.net/update/6/Thunderbird/102.2.2/20220906224751/Linux_x86_64-gcc3/en-CA/release/Linux%205.15.0-139-generic%20(GTK%203.24.33%2Clibpulse%2015.99.0)/ISET:SSE4_2,MEM:15386/default/default/update.xml?force=1 AUS:SVC Checker:checkForUpdates - sending request to: https://aus.thunderbird.net/update/6/Thunderbird/102.2.2/20220906224751/Linux_x86_64-gcc3/en-CA/release/Linux%205.15.0-139-generic%20(GTK%203.24.33%2Clibpulse%2015.99.0)/ISET:SSE4_2,MEM:15386/default/default/update.xml?force=1 AUS:SVC Checker:onLoad - request completed downloading document AUS:SVC Checker:onLoad - Getting sslStatus failed. AUS:SVC Checker:onLoad - number of updates available: 1 AUS:SVC getCanApplyUpdates - testing write access /opt/thunderbird/update.test AUS:SVC getCanApplyUpdates - unable to apply updates without write access to the update directory. Exception: [Exception... "Component returned failure code: 0x80520015 (NS_ERROR_FILE_ACCESS_DENIED) [nsIFile.create]" nsresult: "0x80520015 (NS_ERROR_FILE_ACCESS_DENIED)" location: "JS frame :: resource://gre/modules/UpdateService.jsm :: testWriteAccess :: line 519" data: no]

Thank you once again.

running Linux Mint 21.3 - everything up to date. Thunderbird installed using Synaptic.

Then the Thunderbird built-in updater won't work, you'll have to update using the packet manager/repository provided by your distribution. You may better ask in an Ubuntu forum.

If you are not getting any more Thunderbird updates (and other package updates) from Mint in your package manager then your version of Mint may be EOL in packages support.