No-one is replying to this. Can anyone tell me why or suggest anything else I can do to get help with my problem?

Go to the help menu and click troubleshooting information. Select the show profile button in the profile section. Close Thunderbird Delete the file Global-message-db.sqlite Restart Thunderbird and wait for the search index you just deleted to regenerate. (that might be quite a while)

Many thanks for that, Matt.

After following your instructions, the search now shows the particular message I couldn't find when I raised the question, so I must presume it has fixed the problem.

Not sure why this happened to me. If it's something that just happens from time to time, it might be worth having your solution available in searchable Thunderbird help information. I looked there before asking this question but couldn't find anything.

clicking "this solves it" on the forum allows the topic to be indexed by Google. as for the search internally. I really have given up on that. it is probably the only search in the world that get more matches the more criteria you put in. "Thunderbird crash" with out the inverted commas returns everything with Thunderbird and every thing with crash and just occasionally by accident those with both. So it is about useless really.

Point taken. It's a shame. Maybe a domain-specific Google advanced search would work better.

I'll bear this in mind anyway next time I'm looking for something.