Mines play a big part in PMOG

A new type of multiplayer online game has been developed to play in the background while users surf the web.

The PMOG, or "passively multiplayer online game," is installed as a Firefox add-on.

A user's objective is to build "missions," or grouped websites that follow a theme. For example, "Tech News Tour" includes Engadget, Gizmodo, Digg and Slashdot.

Users can leave mines for other players on websites and buy armor to prevent damage. After achieving high levels, features are unlocked — the ability to choose one of six character types, for example, says TechCrunch.

Developed by GameLayers, the PMOG is in private beta. Its public debut may give advertisers the chance to orchestrate sponsored missions with winning incentives.