Try Compacting message folders, limiting synced number of emails in acc Settings, and switching off Unified View to find the reason. You may also need to Repair some acc folders from left pan RMC menu.

Thanks for those suggestions. When I asked it to compact, it began searching again. When if finished I switched View from Unified to All.

I don't see how to limit number of synced emails (but I have a small number of messages in total - I delete almost all after reading and empty trash frequently, apart from about 40 saved emails).

I also don't know what the RMC menu is.

Note that everything works properly except for the "searching" that it does upon opening the program. Once if finishes that, it gets my messages as it used to do. What is it searching for and can this be turned off?

RMC is right mouse click (Properties-Repair Folder). Message Limits are also accessible there. As well if this is Gmail acc, switch off IMAP access to All Mail label in Gmail webmail Standard View Settings-Labels.

Okay (When I had Googled RMC menu, I got meal options at Royal Military College). So, I compacted folders, limited messages, and in Gmail webmail there were two options to the right of All Mail. I selected HIDE in the first column and in the second column I de-selected Show in IMAP.

It doesn't appear to have solved the problem. The fact that it properly checks for email when I tell it to (versus when it is supposed to check by default upon opening the program) makes me think the interaction with the email providers is okay and interference is coming from some other aspect of Thunderbird.

Anyway, I can certainly use it as is. Maybe it will be solved by a future update (or maybe it is a mysterious setting in my app.).

Thanks for your suggestions.

Search delays are often caused by corrupted Tbird folders, or too many messages in some acc folders in Tbird. Try fully deleting and adding again your accs, messages, and Tbird profile.

Failure to check for new mail at startup can sometimes be fixed by doing this: Help/More Troubleshooting, Profile Folder, Open Folder, close TB, delete or rename session.json.

Deleting session.json. seems to have been the solution in this case. Thanks!