Here was an interesting problem, what was wrong with the following code, which works fine in IE but not in FireFox

<map id="mymap">
<area ... />
<area ... />
</map>

<img usemap="#mymap" ...>

It appears that FireFox doesn't understand the id attribute and the name attribute is required.  According to W3C recommendations, http://www.w3.org/TR/REC-html40/struct/objects.html#adef-usemap the usemap attribute should match the name attribute of the map tag.

Try this instead:

<map id="mymap" name="mymap">