Since firefox-esr on oldstable (Debian 11 / bullseye) was upgraded from
128.8.0esr-1~deb11u1 to 128.9.0esr-1~deb11u1 earlier this month, Firefox
browser windows no longer have distinct icons on the Xfce taskbar --
they always have a generic white-window-with-blue-titlebar icon. This
also affects Xfce's Alt-Tab switcher.

Prior to this upgrade, the icon used for this was the window's current
tab's favico, or failing that, (IIRC) the orange Firefox logo.

I have tried starting firefox-esr with a fresh profile (with no
extensions) and -safe-mode, and the problem persists.

(I saw #1102344, but I'm unsure whether the missing file claimed there
has anything to do with the symptom I'm reporting.)

bugs.debian.org:

Same issue on my machine when I upgraded today:

2025-04-26 12:45:44 root@laalaa ~/laalaa.tracy.holgerdanske.com
# cat /etc/debian_version ; uname -a
11.11
Linux laalaa 5.10.0-34-amd64 #1 SMP Debian 5.10.234-1 (2025-02-24)
x86_64 GNU/Linux

2025-04-26 12:46:39 root@laalaa ~/laalaa.tracy.holgerdanske.com
# dpkg-query --list firefox-esr
Desired=Unknown/Install/Remove/Purge/Hold
|
Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name           Version              Architecture Description
+++-==============-====================-============-============================================================
ii  firefox-esr    128.9.0esr-1~deb11u1 amd64        Mozilla Firefox web
browser - Extended Support Release (ESR)


David

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.)