I believe the index for your Inbox folder is corrupt, which makes it mis-report its contents. To fix it, right-click on the folder name in the right-hand Folders pane, then choose Properties > General Information tab, then click the "Repair Folder" button, then OK. The phantom messages should be gone now.

It wouldn't hurt to compact the folder, too.

You said, "It wouldn't hurt to compact the folder, too." Can you provide some details on how to do this?

Also - it it considered a "good idea" to "compact" my Inbox folder?

Thanks!

Here's a good article talking about compacting folders and why it's a good and necessary thing:

https://support.mozilla.org/en-US/kb/compacting-folders

The short version is that you right-click on the folder name in the Folders List, and choose "Compact". This will make the folder file on the disk smaller and more efficient. You can configure this to happen automatically or to prompt you when a threshold is reached. The setting is found at <Settings | General | Disk Space>. I have mine set at a pretty low threshold, 60 MB, but to prompt me so I can decline if it's an inconvenient time, like I'm shutting down.

I read the article - thanks for that! Just one question:

How do I know whether I have "Mbox" or "Maildir" folders?

OK - I think I found the answer: mbox. Maildir is said to be "better", but according to some sources it remains in an "experimental" status... I think I'll wait...

~S