jimunderscorep,
thank you for your comment. I did as you said and now the pop-up "Please disable your adblocker message is gone! Thank you.
Or install some regular adblock extension like ublock origin 
On Firefox, I have uBlock Origin installed. But wouldn't Opera's speed be hampered by downloading an adblocker extension, as opposed to just subscribing to the text file you helpfully linked to?
I do not know what is different between the 2 lists you mention.
I don't know much, but I do know that the Adblock Killer list/textfile is better with at least two websites. 🙂