NB: The "do not track me' option in FF only works on those websites that voluntarlily participate ... those that do not will track every search you make ... the good guys and the criminally inclined. To me this option is not worth much. Google tracks everybody, not because they are trying to do you harm; they want to sell more advertising and in the sales/marketing world of applied logic, the user searches on stuff that they are interested in. Therefore, the user wants to purchase something.
Extensions on browsers are not vetted by the browser suppliers. . That is, for example Chrome, IE and FF (and others) do not warranty that the code will be malware free or even safe to run. If they get a slew of complaints they will take the extension down. You get to do the cleanup after the fact. However, there are some extensions that have a proven track record for being safe. NoScript and WOT (mentioned here in other posts) are in good standing. I use them myself.
Just a note on tracking: If you download recommended applications from a forum member or from any application downlaod site it could have a tracking website imbeded in it. The number one offender is 'OpenCandy' . It is not considered malware by many security applications as it does not write vicious code on your PC or follow up with any illegal activity ... it just spies on you and calls home about your PC activity. Its purpose is to provide info back to advertisers ...yes, to get you to buy something. NB: Microsoft flagged it as malware in MSE quite some time ago, but most other security applications do not flag it as such.
IMO, the best way to keep a browser safer is to sandbox it. Extensions have their warts and can have problems keeping up with constant browser release updates (FF release updates come in swarms).