Not sure if it is just me, but I can't post in the forums using firefox. I can put in a subject but the body of the message has no place to type it in. It works if I switch to chrome. Maybe somebody can enlighten me or take a look at the forum?

 I'm on Mint using Firefox 80.0.1 and as you can see, no problems.

Manjaro and Foxi 80.0.1 ... works

 Mint-20 Firefox80.0 works fine.

I'd try
go to firefox settings.
Privacy and Security.
Find the "Clear Data" option.
Clear all the cached web content.

If that doesn't work maye an update is needed?
sudo apt-get full-upgrade

But suffice to say as it's working for all of us that it must be a problem on your end not with the forum.