I have also noticed that Thunderbird has stopped defaulting to my Inbox on Startup. Instead it goes to the Sent folder in Local Folders every time even though there are no unread messages in that folder. If I move to the Inbox and Refresh, then the new messages come in. My Account Setting is correct--set to the right account which is in bold and underlined and Inbox is set to be the default folder for getting messages. Default setting in Account Actions is greyed out. All is as was and normal. Does anyone know what is happening here?

For the problem of getting new mail at startup, see this topic:

https://support.mozilla.org/en-US/questions/1275606#answer-1299178

For the startup folder, see this topic:

https://support.mozilla.org/en-US/questions/1292992

I have solved my problem. Here are the steps I took.

1. I went to Sent folder's Properties>Repair.

2. Got a message that the folder was in use by another process (?) and couln't be repaired.

3. I went to Inbox. Nothing happened.

4. Closed TB while in Inbox and reopened TB.

5. TB went to the Sent folder again

6. I navigated to Troubleshooting>Places database>Verify integrity>closed Troubleshooting

7. Went to Sent folders Properties>Repair. No error message this time.

8. Went to Inbox, closed TB again while in Inbox and reopened again.

9. TB went to Sent folder again.

10. Clicked on Inbox and this time got a message that my account was being processed (?) and was unable to get new mail.

11. Closed TB and reopened again. TB went straight to inbox and all new mail came through!!!

TB has worked perfectly since. Goes right to Inbox and gets new mail everytime.

So, do any contributers know precisely what I did that solved the problem? It could help others with the same problem.

The action of 'Verify integrity' is displayed after the process runs, but I don't know why the integrity of places.sqlite would affect your issues, as the places file simply records browsing history. Rebuilding the global-messages-db.sqlite file is usually more relevant to profile issues.

As for the 'other process' that appeared, I don't know if that was an internal TB process, such as folder indexing, or one due to an external app such as antivirus scanning the folder. Tools/Activity Manager or Tools/Developer Tools/Error Console might provide a clue.

Thanks for your suggestions. I have no idea what I did resolved the problem. All I know is that trying to 'repair' the folders alone didn't work and clicking Verify Integrity did. The Tools options you mention were both blank. The problem existed over several days, so I am sure there was no external app processing TB all the time. Remains a mystery why it happened, and as you say, why Verify Integrity appears to have worked. I see several other people have had the problem, and there does not appear to be an agreed diagnosis of the problem or how to solve it. Very frustrating.