Here is a link to a well regarded standards page for the element z-index, http://www.w3schools.com/css/pr_pos_z-index.asp, unfortunately it appears the developers of FireFox (version 3 as of this writing) are not exactly following the indicated example value.
How do I know this? You really have to wonder when Microsoft’s Internet Explorer works and the other three browsers do not. Something has got to be wrong, and so my searches brought me to the conclusion of setting my z-index values to positive values where needed. Internet Explorer can accept and resolve correctly with negative values. It’s not often a Microsoft negative is a positive result.
Have you stumbled on, across, over or just simply ran into one of these CSS walls? What was it, and how did you fix it?