Well, it looks like a settings change in Firefox helped. It's deep in the settings and you'll have to change it manually in about
:config. The setting is:
editor.use_div_for_default_newlines and it should be set to
False to make the forum carriage return work correctly.
Use this at your own risk. Changing the setting in the config editor can void your warranty and cause unexpected issues with Firefox. Always keep the setting handy in case you need to change it back. Furthermore, I have not tested this change to see if it affects other text editor in other forums or websites. It might. Again, use at your own risk and don't say I didn't warn you.
Here is the link to where I found this change:
Firefox inserting extra blank line between paragraphs • mozillaZine Forums
BTW, I made the change and I didn't have to edit this post to remove extra line breaks.