Mozilla Blog: Block slow, privacy-invading content, not "advertising"

tl;dr -

* Content Neutrality: Content blocking software should focus on addressing potential user needs (such as on performance, security, and privacy) instead of blocking specific types of content (such as advertising).

* Transparency & Control: The content blocking software should provide users with transparency and meaningful controls over the needs it is attempting to address.

* Openness: Blocking should maintain a level playing field and should block under the same principles regardless of source of the content. Publishers and other content providers should be given ways to participate in an open Web ecosystem, instead of being placed in a permanent penalty box that closes off the Web to their products and services.