My web browser does this, as well; No matter which one I use, I always get a little pop-up before signing in. Once I click out, though, and exit the window, everything is fine. I can safely say that I haven't gotten a virus from it yet. c:
I wouldn't try to get adblock to remove the ads, because those are what keeps FH running by paying for the server. We have to have ads, or else the game wouldn't exist. As long as you close out, it should be fine. I would make sure you have a generally strong malware system set up, either way, but the pop-up, as long as it's not clicked, shouldn't affect your computer or gameplay.