To confirm you are looking in the right place, I have attached an image showing where the option is.

If it is not there, add-ons can change what is displayed in the options window. To check to see if an add-on is causing your problem use the procedure in this link - https://support.mozilla.com/kb/Troubleshooting+extensions+and+themes

Note that selecting that "do-not-track" setting will only make Firefox send a specific HTTP header (DNT=1) to a web server.

It is up to the server to do something with that request.

• http://blog.sidstamm.com/2011/01/opting-out-of-behavioral-ads.html
• http://blog.sidstamm.com/2011/01/try-out-do-not-track-http-header.html

• http://browserspy.dk/donottrack.php

in german firefox, the settings are a little different to activate dnt

starflame

That's where that preference is located in Firefox 5.0 versions. Firefox 4.0 has that pref in Advanced > General, as TonyE posted.