Mozilla is experimenting with an interesting personalization experience, suggesting a new Firefox API that would collect browser activity and provide it to websites so that they could prioritize content based on your interests. For example, if you've been reading a lot about cars recently around the internet, you could provide this information to a news website that would, in turn, show you a range of car-related articles at the top of their site.
One aspect of the proposed API would be user control, with users being able to select the amount of information shared with websites; if you're concerned about privacy, you could disable the feature and share no information. Mozilla has always been a supporter of user privacy on the web, so while it's a little unusual to see a proposal such as this coming from them, they are being forthcoming about what they're trying to achieve while giving the necessary privacy controls to the user.
The idea is just a proposal at this stage from Mozilla Labs, and with the help of "user researchers" the team has begun testing the concept with a range of volunteers to see how they respond to personalized content. They're also looking for feedback on the idea, asking people to head to the Labs discussion group if they have anything to say about it.
As the project is still in the very early stages, it may be be a while before we see where this goes. The idea also hinges on web content producers using the new API, so it'll be interesting to see where this goes in the months and years to come.