I have been reviewing this for some time now. It appears to be a Firefox issue that came with the last update. Unfortunately, I have yet to find a fix on my end. They do however have a fix for the IP.Board apparently but some have even said that hasn't solved the issue. Pubby, can see what he thinks about it.
For now, with the Rich Text editor disabled I can edit and make replies without having to resort to using the terrible IE browser. The CON is that my response is a little less user friendly with the HTML coding mixed in.
Pubby it might work: http://community.invisionpower.com/to