mass * velocity. Velocity, as you might recall from high school physics, is a "vector" quantity. That means it has a magnitude and a direction (if you're driving the speed limit, your speed is 55 miles per hour; your velocity is 55 miles per hour due north).
User momentum is
user-base size (mass) * workflow (velocity). Firefox, as an application, has a fairly large user base. This user base is biased towards more technical minded types, people who know their way around a computer (let's face it- your average luser is going to just use IE). A technical user is the kind that tends to develop a consistent workflow.
The "AwesomeBar" haters are those kinds of users. They've developed a specific way of working (direction) and gone far down that path (magnitude). They're a subset of the overall Firefox user-base, but big enough, with enough magnitude on their workflow that they've got a lot of momentum.
Newton's First Law tells us that "an object in motion tends to stay in motion unless acted upon by an outside force"; e.g. momentum can only be changed by force. How does one apply force in this case? Well, Mozilla hit the nail on the head: they took away the old address bar behavior entirely. There's no legacy setting, no way to disable the behavior (without disabling all address bar behavior). That's force.
And Newton's Third Law tells us that "for every action there is an equal and opposite reaction". The reaction, in this case, is backlash. "This sucks, I'm going back to FF2." "Mozilla's driving users like me away." Hundreds of angry postings in hundreds of blogs rattle their "I'm dropping Firefox" sabers.
Now, imagine if you will, a train moving down the tracks. You stand on the tracks and attempt to apply force and stop the train. Obviously, you're going to lose: the change in force you have to apply must exceed the momentum of the train (mass * velocity). You can't stop a train. But you can stop a rolling bowling ball. Or a thrown football.
The important question here is: does Mozilla have enough force to counteract the user momentum? Is Mozilla stopping a train, a bowling ball, or a frisbee? In this case, I think it's a frisbee. I think the largest mass of the users (myself included) look at this AwesomeBar and think about how we can work it into our workflow. Either the direction of our workflow isn't too far from what the AwesomeBar tries to do, or the magnitude of our workflow isn't so great. We're willing to change. A subset of the total user base has a great deal of momentum, and there are only two options to deal with them: you apply force to bring them in line (remove legacy compatibility) or you cut them loose so they don't drag your project behind them.
At any rate, after work today, I'm probably going to put together "Remy's Laws of User Motion" to better explain how user behavior can be managed using physics as a metaphor.