I would say it is not a bad thing to use, I tried using Webmail Ad blocker for various streaming sites on Firefox and it was supporting it quite well. But still when I tried using it for few other movie streaming sites like fmovies it could not stop the ad-popping up. Though it did stop the website from creating more ad's on the same page as google chrome does it.
Over all I would say it works very well with normal day to day sites and also ask you if you are sure if you wanna go ahead with this website, but i could only tell less because this was the first time I used Webmail Ad Blocker.