They want to sell user data, plain and simple. "Improve user experience" is the standard excuse. Basically they're saying we want to collect a bunch of information we already know people don't want us collecting, so we want to make it opt-out and we'll pinky-swear it will be kept anonymous. There's no way for them to send data to their servers truly anonymously; there's no way for them to guarantee everyone who has access to the data before it's anonymized will not do something they're not supposed to. They're asking us to move away from a not having to trust anyone to trusting them by default.

I'm sure you already know all this and I'm sure people are getting sick of hearing rants about it every time it comes up. This is the second time in a week for a Mozilla product. I suspect they're trying to exhaust the ranters so they're just left with the users who don't care, "have nothing to hide", or think it's their duty to help the browser vendor squash bugs. No software or service should be trusted until it's absolutely necessary to get the job the user wants done, not the job the browser vendor wants done. It will never be necessary for a browser to send browsing data back to the browser vendor to get to a website.