[johnnywasagood]

Good default is to ask users about their preferences explicitly and not to hide that kind of settings anywhere.

[LtdJorge]

Most users are computer illiterate, so they would choose to not auto-update to skip the hassle, and then never manually update anyways.

[jacquesm]

Every FireFox install comes with auto-update enabled.

[roman-holovin]

Nope, mine on linux doesn't auto-update itself, though I update it diligently, but manually