"As Do Not Track picks up steam and standardization is well underway in the W3C, people have begun asking, "If Do Not Track is so good for the web, why don't you turn it on by default?"
"Frankly, it becomes meaningless if we enable it by default for all our users. Do Not Track is intended to express an individual's choice, or preference, to not be tracked. It's important that the signal represents a choice made by the person behind the keyboard and not the software maker, because ultimately it's not Firefox being tracked, it's the user. "Sure, we could run a few engagement campaigns to inform people about the option, but we won't make that decision for our users.
Edit (9-Nov-2011 @ 11:24):
There are three different signals to consider in broadcasting the user's preferences for tracking:
1. User says they accept tracking
2. User says they reject tracking
3. User hasn't chosen anything
We're defaulting to state 3: we don't know what the user wants, so we're not sending any signals to servers. The signal being sent should be the user's choice, not ours, so we don't broadcast anything until they've chosen what to send.