How to switch off table guide border?

Hi,

If I use chrome or firefox, ckeditor shows a dotted line around table cells. This is a problem as if you have nested tables it makes the design go out of alignment.

I can see this might be a useful feature for some, but it's not for me - or our users who will find this confusing.

It does not do this in IE. No border guides are shown in IE.

Note that the tables I am referring to do not have borders when rendered in html (which is correct).

Is there a way to toggle this feature?
Why is it only doing this in Chrome and Firefox?

I have tested this with the most simple code. i.e. a table with border=0.

Many Thanks
Simon