I've had a similar issue before. If you click on "settings" (top right corner when logged in), then "General Settings" (midway down on the left side), then scroll down to "Message Editor Interface", what is it set to. If it isn't the Basic Editor try switching to that and see if it fixes it.
Sunshines, I know it's a long time ago and I don't know if it'll be recorded (I doubt it but in my search history if confined to Suggs & Feedback) this is the EXACT same problem I had, both that and then the little quick reply box. Now that I use Chrome I've never had that issue.
I didn't clear cache. I'm using windows 7 ultimate 64bit & it's the latest version of Firefox (19.0.2). Everything is back to normal now though (I didn't word it very clear when i wrote my last post). I just started my PC back up on the night and it was working correctly again.
"I won't scatter your sorrow to the heartless sea. I will always be with you. Plant your roots in me. I won't see you end as ashes. You're all diamonds."