I've previously reported (when the redesign was new, so a while ago at this point) that the text boxes for the forum and comments are busted on Firefox compared to Chromium and derivatives.
I think this problem has to do with "padding-bottom: 0px" but I'm not a webdev.
But you can still post comments, it just looks terrible. But I've found that if you go to, for instance, this page:
Then switch from premium video to any other version you can't post a comment at all. The entire input box is unresponsive.
If you do the same thing on Chromium based browsers you can still post comments but something odd happens there too. More or less three quarters of the input box becomes unresponsive. Kinda how the box acts for Firefox at all times except the colour isn't incorrect.