What sign?

• Create a screenshot of the problem

A One Way No Entry Sign

It would certainly help if you include your Thunderbird window for the screenshot, or at least parts of it.

I am having the same issue. Here is a screenshot of it.

I would call this a 'no entry' sign, though it is of course true that these are often found at the mouth of a one-way street.

OK, I've figured it out myself.

This means that the thread is being ignored and future replies will not be shown.

To turn it on and off, select Message > Ignore Thread.

You can toggle it on and off with the 'K' key as well.