"Your browser provides a lot information about your system, for example the browser type and version, the operating system type and version (including major service packs), the browser plug-ins and their versions, the screen resolution and DPI of your display. You add all that together and it makes a pretty good signature."
Just to follow up on this, you can check to see how unique your browser's signature is by using Panopticlick, a tool set up by the Electronic Frontier Foundation (EFF).
There are many excellent tools available for Firefox browser that can assist you in protecting your privacy. Here are a few of my favorites:
- User Agent Overrider: Use this to change your browser's user agent so as to appear as generic as possible. You can get a list of the most popular user agents here.
- Adblock Edge: Use EasyList to block ads, EasyPrivacy to block trackers. Add the Fanboy social list to block Facebook buttons, etc. Social media sharing buttons track your browsing even if you don't have an account with them.
- Self-Destructing Cookies: Accepts cookies from Web sites but deletes them automatically when the tab you're using is closed unless you say otherwise. Fights tracking cookies.
- HTTPS Everywhere: An extension by the EFF. Enables secure connections whenever possible.
- NoScript: One of the best security and privacy tools that Firefox has to offer. NoScript has a significant learning curve, but if you master it, you can stop most threats cold. For example, try enabling NoScript and then going to Whatismybrowser. You'll see that it is impossible for that site to get a list of the plugins you have installed.