does the about:Thunderbird show the version as 64bit? Is your operating system 64 bit?

Thanks for responding Matt - the answer is yes to both questions.

Thanks for responding Matt - the answer is yes to both questions.

That is a bit sad really as I had half formed a theory that some of the false update mesages were coming from 32bit Thunderbird on 64bit Windows.

I suggest you try using the installed from Thunderbird.net and see if it reappears in 4 weeks when the next security update comes round.

At least some of these are odd things coming from anti virus programs alerting to updates that the anti virus provider has flagged that Thunderbird is not offering at the time. But those mostly occur with full version updates, not security ones. But there is no real "this is what causes it and this fixes it" for this subject.