Bug#1104148: firefox-esr: no taskbar icons since upgrade to 128.9.0esr on bullseye

[Jacob Nevins]

As of today's bullseye update (128.10.1esr-1~deb11u1), all my Firefox
windows now have the Firefox icon rather than the generic-window icon,
which is a great improvement.

(And now I'm doubting my recollection that websites' favicons were ever
used.)