I encountered that container "overflow" problem a while back. In my skin's css, the contentpane should have a width property (for IE) and a max-width property (for FF). It should look like this in your SKIN css:

div.LeftCell {
width: 25% /* for IE */
max-width: 25% /* for FF */
}

max-width will be ignored by IE.

and then in my container's css:

div.ContentPane {
  width: 100%;
}

and then they all play nicely. Let me know if that works, if not, I tried! Sorry!

-Rose

To follow up:

I ended up re-writing  the skin css to suit Firefox, but then it looked awful in IE, so I used the bit of javascript that is in the CSS skin at DNNcreative.com to make the skin use a different CSS stylesheet for IE. So now I can live with the skin in all three browsers.

Opera - to its credit was happy to render either CSS stylesheet almost identically. God bless it.

Firefox styling seemed to be all about taking account of padding when setting widths for inner divs. 

Cheers all who replied.