Flock, the so-called “social browser” built on top of Mozilla technology, has raised $15 million in a Series D round led by Fidelity Ventures and joined by Bessemer Venture Partners, Catamount Ventures, and Shasta Ventures.

The round (the company’s biggest) more than doubles its total to over $28 million, an amount that has been gradually raised over the past three years even though Flock 1.0 launched only this past November.

CEO Shawn Hardin speaks about Flock’s mission in very sweeping terms: enabling users to express themselves, participate in online communities, have voices, and engage their peers. As he sees it, the web is experiencing a paradigm shift from consumption to participation, and it needs a new type of browser to go along with that shift.

Flock is basically a suite of browser extensions with ties into web services like Facebook and Twitter. A personalized homepage called MyWorld and a special sidebar serve as feed readers and friend update aggregators. You’re given quick access to Gmail and Yahoo webmail accounts and any blogs that you administer. And a media bar along the top makes for quick searching on Flickr, YouTube, and other social media sites.

When asked whether regular browser extensions pose much competition for Flock, Hardin suggests that very few people actually enjoy personalizing things enough to set up the breadth of functionality provided by Flock. Plus, Flock already has a proven revenue model where these do not; it earns money the same way Mozilla does, through search placement deals with the engines (Yahoo and a few others in Flock’s case).

Almost 4 million people have downloaded Flock, and users are said to use it for over 4 hours per day on average.

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