It's not a disruptive ad, or at least not obviously...
I am using Firefox 50.0 on a MacBook running MacOS 10.11.6. Activity Monitor reports CPU activity in the 90 to 145% range for Firefox when I have a tab or tabs with MyFlyerTalk or any FlyerTalk forum page open. If I close all tabs with MyFlyerTalk or any forum, CPU activity drops to 50-75%. If I close all FlyerTalk tabs, the Firefox CPU activity drops to less than 5%. This is consistent, and it is only with FlyerTalk. Also, the excessive CPU activity continues even if I turn Wi-Fi off; I need to close the FlyerTalk tabs. Any ideas?