AdBlock Plus is a controversial Firefox plugin to prevent ads from displaying on web pages. There is a simple procedure to detect (and take appropriate action) AdBlock Plus.

The idea is to dupe itself in thinking that a Javascript is from a well known advertising site like Google AdSense or falkag.net. Here is a code snippet that I found on AdBlock Plus forum that should give you the essential idea:

< script language="javascript">
var disabled = false;
< /script>
< script src="http://www.avforums.com/something.js?thisistotrickyou=http://a.as-us.falkag.net/dat/njf/41/domain.com/ros_pop_tag.js">
< script language="javascript">
if(!disabled){
alert("You Are Using Ad Block Plus or some other advert blocking software!\nAVForums relies on advertising for revenue. Please add AVForums to your ad blocking whitelist or disable ad blocking when you visit AVForums.\nFor more information, visit http://www.avforums.com/forums/view_advertising_policy.htm");
}
< /script>

There are three issues with this code snippet:
1. It doesn't specifically detect AdBlock Plus.
2. Dedicated users of your site may use regular expression to specifically exclude such url's on your site.
3. AdBlock Plus may be modified in future to catch this pattern

You can use PHP / JSP to regularly change the URL pattern, switching between alternative URL's. This will address most of the issues. I may even post an example if there is enough interest.