We are still about a month out on the next Fx build (22.214.171.124). One of the improvements with this build is going to be major improvements to ad blocking using the Adblock Plus extension. A couple patches have been added to the Gecko 1.8 branch which will work with the upcoming Adblock Plus 0.7.5:
The patches ensure that content requested by plugins like Flash and Ajax based applications will be subject to the same content policies as regular web pages. So if for example you have asked to block images from server “http://ads.server.net” they will be blocked even if requested by a Flash application or an XMLHttpRequest, one of the core functions used by rich Ajax-based web applications.
Yahoo! Maps, for example, embeds ads in its Flash-based application. These ads are not subject to regular content filtering and so, are displayed even if using an ad blocking extension like Adblock Plus.