Firefox (version 22, I haven't tried this in earlier versions) cannot somehow find the reference to a .js file where my "dropdownmenu()" javascript function is located. Here is how I tested this: I created a "myalert()" function and a "dropdownmenu()" function that are both identical and look like this (just for testing purposes): function myalert() { alert('javascript executed'); } I put "myalert()" javascript on the aspx page itself. I put "dropdownmenu()" function in a separate .js file in the "scripts" folder. I tried the following two methods of referencing that .js script from the aspx page: <script src="scripts/actionmenu.js" type="text/javascript"></script> <script src="http://localhost/scripts/actionmenu.js" type="text/javascript"></script> The aspx page has the two hrefs (to the two javascript functions) defined as follows: <a href="javascript: dropdownmenu();" class="hovermenu" > My Options</a> <a href="javascript: myalert();" class="hovermenu" > My Alert</a> Clicking on "My Alert" works fine. So, if the javascript function is on the aspx page itself then everything works fine. Clicking on the My Options does nothing. So the javascript defined in a separate .js file does not get called. Just to verify that I specified the correct location of the .js file, I opened the page in IE and clicked on My Options. It worked fine. It was able to find that "dropdownmenu()" function in my .js file. So the location of the .js file is definitely specified correctly but for some reason Firefox cannot find it or access it.