Check new Firefox favicon code for first-party isolation
mcs and brade noticed some new favicon patches in Firefox that we should check for first-party isolation, and patch if FPI is violated: https://bugzilla.mozilla.org/show_bug.cgi?id=530999 https://bugzilla.mozilla.org/show_bug.cgi?id=1184739 https://bugzilla.mozilla.org/show_bug.cgi?id=1206560