“If you’re getting the following error
Failed to post. Please try again.
check to see if you are running the following add-ons in Firefox: uBlock Origin, Privacy Badger, and HTTPS Everywhere. If you are, read on.
To bypass this error, I opened a new private window, disabled the 3 add-ons and it miraculously worked.
Hope this helps somebody out there. I spent a long time searching for an answer before I stumbled on this solution.