Reply to my reply.
I also use Firefox and am not having problems typing now, but did for a while. This message "Your message should be 10 characters, etc" is not the result of a short message - they are plenty long enough. It seems to me it has to do with where you start your reply. I just did one from the top - with the message I'm replying to on the bottom - I don't care for this, but this is where the cursor is when I hit reply with quote.