Because the choice of an adblocker should be left to the user. Adblock plus and Adblock both make a lot of money by letting some ads unfiltered in the default s

Because the choice of an adblocker should be left to the user. Adblock plus and Adblock both make a lot of money by letting some ads unfiltered in the default settings. That kind of behaviour could not be allowed built in in a browser like firefox. Plus they have the risk of website boycotting the browser, in the same way than website today forbid adblock, but way more powerful because it could be done server side.