Going into geeky web developer mode here:

In a CSS layout, any block-level element that has an outline applied to its :focus state (by default in Firefox and most browsers for accessibility reasons,) and that is also enclosed in a container element that has overflow: auto applied to it (a widely used float clearing method) will potentially break the layout on focus.

The fix for this is to include :focus { outline-offset: -1px; } (or a similar value) in your stylesheet. This fixes the issue in Firefox 3.5.2 Win. More testing to come soon (Opera.)