I downloaded this add-on for the first time a couple weeks ago and was terribly disappointed. Less than 25% of ads were being blocked, and I had to manually keep adding filter rules to block ads, which was slowing me down online (and add-ons are supposed to ENHANCE our browsing, not drag us down, right?). I was so frustrated I nearly abandoned Firefox altogether in favor of Google Chrome (because the AdBlocker I downloaded for Chrome worked perfectly--however I hate Chrome's Download Manager, but I was so disgusted with FF's AdBlock Plus that I was ready to put up with Chrome's weird downloads).
Anyway, I thought I'd give things one more try. So I uninstalled both AdBlock Plus AND Firefox, then reinstalled both. When I chose my filter list, I chose EASYLIST instead of the FANBOY LIST (or whatever it's called), which was the default choice for me. And VOILA! Now AdBlock Plus works PERFECTLY!! I am so thrilled! Now I know what all the rave reviews here were about: AdBlock Plus is fantastic...IF you use the EASYLIST filter and not FANBOYLIST!
So...if any of you are having problems with ABP or it just doesn't seem to be working as promised, check your filter subscription: make sure you check EASYLIST English (for U.S. users). It made all the difference in the world to me...I LOVE IT!!!