Make sure you have set up Junk Controls correctly.

• http://kb.mozillazine.org/Junk_Mail_Controls
• https://support.mozilla.org/en-US/kb/thunderbird-and-junk-spam-messages

Under

Tools|Account Settings|{select account}|Junk Controls

have you told it where to move Junk messages to?

Thanks. My junk controls are set for "Move new junk messages to ... junk on local folders". And, indeed, if I mark a message as junk, then it is moved to that folder. But, if Thunderbird decides that a message is junk, it comes to my inbox (and not to the junk folder, where I want it to go). I've tried checking things on junk controls, and it doesn't route these messages to junk. I also read the piece on junk on support.mozilla. I'm sure this is something silly, but I can't seem to crack it!

• Does Thunderbird work in TB Safe mode (see Thunderbird Safe Mode)?

I found the problem. I had to check "mark messages determined to be junk a read". (I thought that would merely make the message heads be in regular, rather than bold-face type. But, actually, it is the box that gets the messages, identified as junk, moved automatically to the junk folder.)

Thanks, all, for your help on this. Sorry I was so dense . . .

No, no, thank you for reporting back. It's not obvious why that should make any difference.

I get unread messages correctly stored in Junk. But it's possible those are messages I'm putting there via another client on a different device. I need to pay more attention.