Mark Surman, Mozilla's executive director, said: "It's a stake in the ground in terms of letting people know the ways they are being tracked. At Mozilla, we believe everyone should be in control of their user data and privacy and we want people to make informed decisions about their web experience."
Mozilla already offers users the ability to disable "cookies" - small files that download from websites on to a computer, allowing advertisers to target users based on their online activity.
Lightbeam will reveal the source of the third-party adverts, scripts and images stored on a web page which are linked to servers in other domains. An expanding graph visualises the interactions between the sites a user intentionally visits and the third parties which may not be welcome.
Mozilla had come under "tremendous pressure" from trade bodies over its mission to bring transparency to the web, said Alex Fowler, the company's privacy officer.
The company said it was responding to increased privacy concerns following the revelation that the US National Security Agency had tapped into the servers of internet firms, including Facebook, to track online communication.
- Independent