If you're a bit concerned that services associated with your Google, Yahoo or Microsoft account are increasingly using your e-mails and online activities to target advertising at you, then Targeted Advertising Cookie Opt-Out (TACO) is a handy Firefox extension to block them.
TACO prevents over 100 different online advertising networks - including Google, Yahoo and Microsoft - from displaying highly targeted advertisements using the detailed information on your web surfing habits. TACO works by placing "opt-out" cookies into Firefox which prevent the services from targeting you with ads.
It is a pretty drastic tool. TACO will clear-out all of your existing cookies except the 100 it places in Firefox meaning you'll lose a lot of your stored passwords, filled-in-forms, etc.
TACO does give you much greater awareness of which sites are trying to track you however. You can now see how you are being tracked at every website and privacy alerts show you which trackers are active. It also blocks many tracking techniques such and network scripts, widgets and other web bugs.
Some features are still in beta testing such as disposable email generation and other protection tools but these are disabled by default. Finally, the developers claim that they do not track users that install TACO and to encourage confidence in the add-on, they've also issued the source code for all to see.
TACO has become somewhat bloated over the years but its still an excellent away to stop advertising from tracking and targeting you when using Firefox.