So here's a tip: the workaround to get proper search back is to rightclick on the folder you want to search, then choose Search:
This enables you to search just one folder of one account e.g. the Inbox. Which is much faster than the toolbar search box, and oddly enough it does search everything properly. The two types of searches must be using different indexes, somehow.
Unfortunately I still can't figure out how to search all the folders of a single account only, i.e. the way the old Thunderbird search box used to work.
A not very satisfactory workaround is, in the new search window that comes up with the right click, to then choose another folder in the "Search for messages in" dropdown. And so on. Which mean you lose the previous search results, of course.
If anyone knows how to get Thunderbird to search all folders of just one account, please tell me. My "desperation" solution now when I need do do that is simply to login to the account in Gmail in my web browser and use the standard Gmail search there!
The big Thunderbird update before this one killed my computer by indexing everything constantly by default. To stop that, go to Tools, Options, Advanced, General, under Advanced Configuration UNtick "Enable Global Search and Indexer", and OK).