In my new job I recently discovered what seems to be a bug in Firefox 3.5: when there is a link (or anything else to which an outline could apply, i.e., any element with an :active state or a tabindex) that is floated (i.e. float: left or float: right) inside a container with overflow set to auto, the outline on that link will take up space, contrary to the W3C spec. This means that such a link, if it fills the container in either the X or Y dimension, will cause scrollbars to appear when it is active (tabbed to or pressed).
But there’s a fix: If you apply CSS outline-offset with a negative value, e.g. outline-offset: -2px;, I suspect it still takes up space (in a sense), BUT that space is inside the element’s current boundaries, so no harm done (unless you really needed that outline to conform precisely to the boundaries.