I presume this is a gmail account. Since all mail is always in the AllMail folder, you need to provide more details on the problem. Thanks.

Thanks! It is a gmail account. Incoming mail is only going to the AllMail folder (unlike before, when everything would also go to my Inbox, unless it was blocked or designated as spam). And there has been a marked reduction in the number of emails coming in even to AllMail. Not sure what might have caused this. I did all the things suggested for gmail: deleted unimportant messages, buying more storage. I am using less than 10% of my google storage at this point. So I suspect it's a Thunderbird issue. I am unaware of changing any settings in TBird, though. The problem started overnight.

I suggest checking your online account to see where the messages are. That is the source and will be a good sanity check.

Yes, I have done that. It is the same when accessed via the web. No new messages are going to my Inbox, just All Mail. Any idea how to fix that?

I suggest you do some experimenting. send a message to yourself and see where it ends up. you may have a filter that is causing this.

Yes, I had done that previously, and the test message only goes to the AllMail folder, like every other incoming messages. And I have not set any filters that would affect all incoming mail...