Tales of Favicons and Caches: Persistent Tracking in Modern Browsers [pdf]
"Firefox: ... However, it never actually uses the cache to fetch the entries. As a result, Firefox actually issues requests to re-fetch favicons that are already present in the cache. We have reported this bug to the Mozilla team, who verified and acknowledged it. At the time of submission, this remains an open issue. Nonetheless, we believe that once this bug is fixed our attack will work in Firefox..."<p>Gosh, I hope the favicon cache bug the authors filed isn't fixed until a broader mitigation against this is implemented.