It's a promising idea but has issues currently. In recent Nightly builds I found it completely broke websites that override control keys [1], which is loads (GitHub, Facebook, Google Docs, etc.). It'll be great when they iron out all the bugs, because this guards against all sorts of the more advanced, sinister ways of deanonymising people.

[1] https://bugzilla.mozilla.org/show_bug.cgi?id=1433592