If you're concerned about advertisers tracking you across the web, Mozilla can now help you see exactly who's following you online with a new experimental Firefox addon called Collusion.
The browser extension creates a real-time graph of all the tracking cookies being deposited on your browser as you move around the web. The addon can differentiate between behavioural tracking (cookies that record links you click on, what content you view, searches you make on a site, etc.) and other potential tracking cookies.
Collusion's graph also makes it easy to see which sites are using the same behavioural tracking advertisers. Collusion was originally developed as an independent project by Mozilla engineer Atul Varma. Mozilla is now developing the addon with the support of the Ford Foundation.
After you've installed Collusion from Mozilla's Firefox addons gallery you have to enable it by clicking on Tools>Add-ons>Extensions and then click "Enable" next to Collusion. After that you should see a small red circle on the bottom right of your browser. Now, just start browsing the web as you normally would. To see the tracking graph build up, click on the Collusion icon in the bottom right of your screen. This will open a separate browser tab with your Collusion graph.
The glowing circles represent sites you have visited and each line growing out of that circle is attached to a cookie the site or its advertisers have placed on your browser. Red circles are behavioural tracking cookies, and gray circles represent non-behavourial tracking cookies. Mozilla says those gray sites may still be tracking you across the web.
After you've visited about four or five sites, the graph tends to get really confusing and it's hard to tell which advertisers are connected with which sites. To cut down on the confusion just hover over any of the sites you've visited and Collusion will highlight only the cookies connected with that site.
You can also do the same for advertisers. If you wanted to see how many of the sites you visit rely on Google's DoubleClick for advertising, you could just hover over the DoubleClick circle. This is helpful information to know since, as Mozilla points out, when the same sites rely on the same tracking cookies, advertisers are able to effectively track you across the sites you visit, building up valuable data for market research.
Mozilla says that all tracking data Collusion collects is stored locally on your computer and never leaves your possession. You can reset the graph at any time to delete Collusion's database. The addon also features an export function, but in my tests this feature wasn't functioning properly.
Collusion can only show you who's tracking you right now, but future plans for the addon include the ability to turn off tracking cookies when you don't want to be followed as you browse the web.