From the article: ...That would leave Firefox in a permanent state of blocking advertisers' cookies by default. Default means this feature would be an opt-o

From the article: ...That would leave Firefox in a permanent state of blocking advertisers' cookies by default. Default means this feature would be an opt-out feature, not an opt-in feature. Since Firefox now autoupdates, that means soon most Firefox installs would have this enabled by default without any user input.