In Firefox 2, in bidi mode, when you have a layout where a non-floated column set up to be a containing block is beside a right-floated column, the layout of the non-floated column gets pushed to the right (overlapping the right floated column). Unfortunately, I don’t have a picture right now, which would help. (I’ll try and take care of that later).
The fix is to add margin-right:auto to your containing block. And this bug appears to be fixed in Firefox 3 – which most Firefox users have updated to, because Firefox users are savvy users and know that pulling the latest browser is a good thing to do.