I haven’t heard of that one before. You might want to check your computer for malware and maybe re-install Firefox. What version of Firefox are you running, by the way?
One thing you can do is edit your reply in a text editor (notepad or whatever) and then copy and paste it into the reply box when you are done. That way if it screws up, all you need to do is copy and paste it again and you won’t lose anything.
Is anyone else having this problem?