I have the latest Firefox, am using Windows 7 and my version of ABP is 3.0.1.
Affected site:
http://agar.io/
Active filterlists:
Adblock Warning Removal List
EasyList
EasyPrivacy
In order for the described issue to take effect you have to click play first. It should transition into a black screen then into a Captcha but I get stuck on the black screen and nothing happens afterwards. There just isn't any Captcha, it's blank.