Actually this question is based on a Blog Entry, which discusses the topic for FF2. But how does this work with FireFox 3?
I know that there must be a workaround, because recently I visited a site saying: "Please disable AdBlock+ before entering this site" :(
There's no general-purpose way to do that, for security reasons. Even the FF2 version mentioned there was specific to that particular extension -- you can't assume identically-named and -placed icons for all extensions.
The AdBlock-spotters, as Stephen presumed, tend to be behavior-based and specific to that extension. Likewise Greasemonkey-spotters, etc.
Please see this url. http://www.guyfromchennai.com/?p=104 This is worked for me
There are other ways they could be detecting AdBlock+. It could tell that you never loaded the ad with some server side code and assume you're using it, maybe?
Or if the ad is flashed based, javascript could try to communicate with it first.
来源：https://stackoverflow.com/questions/571092/how-to-check-with-javascript-if-a-firefox-3-add-on-extension-is-installed