I think I may have found a solution.
First, I found the following thread. It involves using Firefox on other forums, and the symptoms it describes are not the same as what we’ve been seeing here. But it adds credence to my observation that the problem began when Firefox 60 was introduced, and it appears to confirm that the issue involves interactions between Firefox and the characteristics of the specific software that is used by specific forums.
http://forums.mozillazine.org/viewtopic.php?f=9&t=3040337 I’m posting this with Firefox, composed in a normal manner, after first running some tests using the Preview function under the response box, the tests involving changing an obscure Firefox setting which was referred to in that thread. That being as follows:
In the Firefox address bar enter "about:config" (without the quotes). If a cautionary note appears bypass it by clicking "I accept the risk," or whatever such words appear. (I’ve routinely done that many times over the years for various reasons, without issue). In the search box which then appears enter "editor.use" (without the quotes). Right-click on the line that says "editor.use_div_for_default_newlines" and click "Toggle." That should change its value from "True" to "False."
Close Firefox, re-open it, and hopefully you’ll find that the problem is solved, with no side-effects. That appears to be the case for me, as far as I have been able to tell based on some preliminary tests and on composing and submitting this post. And if any undesirable side-effects subsequently appear, here or elsewhere, it’s easy enough to change the value back to "True."
Best regards,
-- Al