The add-on was probably a test. The feature is builtin to Firefox 4 and Firefox 5 In '''Firefox 4''' it's options is in '''Tools > Options > Advanced > General''' * How do I stop websites from tracking me? | How to | Firefox Help<br>https://support.mozilla.com/kb/how-do-i-stop-websites-tracking-me In '''Firefox 5''', see same page, <br>it's option has been moved to '''Tools > Options > Privacy''' This option only lets sites know what your preference is, sites do not have to check it nor comply. I won't be checking it and there is nothing on my pages that store/share anything, so it wouldn't make any difference even if I knew how to check it.