Well, the editor gives this warning because of several issues about integration with Firefox. But, I won't bore you with technical stuff. Here's how to do easy copy and paste on this site :

1. Just use "ctrl+c" and "ctrl+v" (copy and paste respectively) using your keyboard and that should work. Don't ask why this is different, it's some more boring technical stuff I don't have time to mess around with now. I actually never use the mouse for copy-paste so I admit I haven't noticed this before.
2. You'll notice that on the editor you have a button with "T" and a button with "W" next to the smileys. Those are actually two kinds of special paste that allow you to paste text-only or convert formatting (like Word's) to the editor.

Hope this works for you. I will add this "issue" to the ever-growing queue of small things to try and take care of in the future.

Thanks for bringing this up.

Fili