...the reason Firefox was so odd in it treatment of alt/title is that it was the only browser that did what it was supposed to do. (Haven't checked, but I really hope this is no longer true.)
As far as I'm aware, all modern browsers handle this the same now. As I recall it was IE6 and IE7 that were the anomalies (fixed in IE8), in that they would display the ALT text as a tool tip. However, if a TITLE was specified then this would take priority. The problem came when an IMG that had just an ALT attribute was included in an anchor that had a TITLE attribute. IE6 and 7 would display the ALT text of the image as a tool tip, whereas all other browsers would correctly show the TITLE text of the anchor as a tool tip.