what happens if you press alt+3 on the keyboard?

Nothing that I can see.

By the way, another change that has occurred since the update is that the Spellchecker no longer works.

I should say that I am using a W10 machine, and that this is running version 38.5.0 of Thunderbird.

Thanks

I suggest you use a current version that is supported if you want support. I have not used that version for many years now and I am not aware of anyone that is.

The version you have is still using the lightening calendar, an add-on that is not receiving any updates to accommodate authentication changes with cloud providers or security issues.

I was not aware I was not on the current version. Under Help>About Mozilla Thunderbird it reports that Thunderbird is up to date, and that I am currently on the release update channel. However, based on what you are telling me, clearly this is not correct

Are you able to advise how to update to a current version without losing data/history, or point me towards on-line advice?

Yes, the current release version is 102.2.2. Once you don't update the update process decleres you so far in the past there is no valid "upgrade path"

Just go to Thunderbird.net and download the updated version and install it. This far down the track there are almost certain to be some issues arise from the update. Mostly around remembering passwords and should you update to a 64 bit version Thunderbird loosing your existing profile. But neither thing is a major issue really as nothing is lost. Only misplaced.

See https://thunderbirdtweaks.blogspot.com/2019/09/i-lost-my-profilemail-on-update-to.html where I talk about restoring lost profiles post update.

Job done, thanks! It is now on a 64bit version, but I cannot see that I have lost anything. However, the spell checker is still not functioning...