Don't forget, Firefox didn't succeed because Firefox was good, it succeeded because IE was bad. People don't like having to choose between Google Chrome and Google Firefox.
Ad-blockers are not just for YouTube. Moreover, addons can still be a draw for Firefox. For example, the built-in Multi-Container addon, and the ones that build on it, are very useful privacy and web testing tools.
My prediction is that they're going to slowly ban them from their properties, and also from websites where their ads are hosted. They will slowly pressurize Mozilla and anyone else they're funding.