After the revelations came forth of NSA spying this past summer, few companies jumped to action quite like Mozilla. Without much delay, the company patched-up a specific exploit in Firefox that the NSA had taken advantage of, and since then, it's been amping up security in other ways, such as with disabling cookies by default.
With its new Lightbeam add-on, Mozilla furthers its goal to expose those who effectively watch what we do on the Web. The reason the company wants cookies disabled by default is that advertisers and other agencies can use them to effectively target you for more relevant advertising - and because some advertisers are all over the Web, you can understand why some might consider this to be a problem.
It's one thing to understand that you're being tracked constantly, but it's another thing entirely to actually see it. Behold, Lightbeam:
Is that just not beautiful? I don't use Firefox, so to test, I visited 20 of the world's most popular English websites to generate the above, and as you can see, there were a lot of connections being created. The best thing this helps visualize is how one service will effectively track you from one site to the next. Facebook is one good example of this. For the most part, though, most of the sites that you will find share similar connections will be to services you haven't heard of. Connections like those are almost always going to be advertising-related, since actual stats gathering tends to be from known sources (eg: Google Analytics).
If you're planning to run Lightbeam, you'll have the option to contribute your data to a global pool, where an ultimate interconnection map can be constructed, so that we can get the best idea possible of how the Web comes together. It's daunting to think about, but the result could be amazing to see.