OK, that didn't work (didn't see the preview the first time). I'll try again...
When there's an <hr> tag in an HTML message, instead of rendering a horizontal line like it should, TB does something else: it indents the rest of the message, adds a bar on the left, and then hides hides it all as "quoted text".
The really annoying thing is that this only seems to happen with normal <hr> tags that are just part of the received messages body (there is no quoted text in the received message). It doesn't work for top-posted Outlook-generated messages I receive that do contain many thousands of lines of quoted messages. None of that stuff gets hidden.
How do I get TB to render <hr> as a horizontal rule?

Please share the actual HTML of your signature. I tried the hr tag and it worked as designed in my sig. Thank you.

Please share the actual HTML of your signature. I tried the hr tag and it worked as designed in my sig. Thank you.

There's no signature involved. The hr tag is in the body of the message. Though it always "quotes" the content below the hr, it doesn't always hide it. [I can't figure out what triggers the hiding.]

I can't figure out how to add code/html to a forum message, so I guess I'll add images of the HTML and of the way that TB renders it.

Ah! I just realized this seems to be a problem with the "Thunderbird Conversations" plugin.

When viewed in "classic reader" it renders the message properly.

Edit: There's an open bug for this at https://github.com/thunderbird-conversations/thunderbird-conversations/issues/2061