Yeah, that's annoying.  Are you seeing this just in FF, or in some other browser?

Telerik just released an update for their controls, including _this_ editor, that among a whole bunch of fixes and additions adds "Full compatibility with FireFox 3.0 and Opera 9.5" - so that might take care of this problem. 

I dropped their latest into our project today but haven't had a chance to see if it resolves any of the existing issues we've been seeing (Opera 9.5 rendering quirks and this, off the top of my head).  I'll dig around a little more tomorrow...