This may be very hard, if not impossible to do in FF. It would be much easier to even tell, if you post a link to a demo page of this problem. It is much harder to just guess from an image of the trouble. I'd try setting the height for .shadetabs li and/or .shadetabs li a to the height of the image. Another thing I would look into is getting rid of align="absmiddle" on the image and using a relatively positioned span around the link text to position it. So hard to say what will work without the full code and resources (images) of the page.
Please post a link to the page on your site that contains the problematic script so we can check it out.