Recently Mozilla made available Do Not Track option for Firefox 4 users, Firefox 3.6 and earlier versions users can also Opt-out of tracking for behavioral advertising by installing Universal Behavioral Advertising Opt-out add-on.
Install Universal Behavioral Advertising Opt-out add-on to add Do Not Track Header for Firefox 3.6 browser
By enabling “Tell Web Sites I do not wanted to be tracked” option for Firefox 4 browser Firefox sends “DNT:1” HTTP header with all requests, so all servers know the users wants to opt-out, the same will be achieved in Firefox 3.6 and its earlier versions with the install of Universal Behavioral Advertising Opt-out (UBAO) add-on.
Firefox 4 users don’t need to install this add-on as Firefox 4 provides built-in Do No Track option. AdBlock Plus and NoScript add-ons also sends DNT header but if you don’t want extra features that comes with those add-ons you can prefer UBAO.