I notice routine runs of the below always catch xulrunner.

# set --  `deborphan ` && test $# -gt 0 && aptitude purge $@
The following packages will be REMOVED:
  xulrunner-16.0{ap}
0 packages upgraded, 0 newly installed, 1 to remove and 16 not upgraded.
Need to get 0 B of archives. After unpacking 28.3 MB will be freed.
Do you want to continue? [Y/n/?]
(Reading database ... 133541 files and directories currently installed.)
Removing xulrunner-16.0 ...
Processing triggers for man-db ...

Maybe something could be changed so the old one will be purged upon the
next new one.

src:iceweasel has been superseded by src:firefox-esr in version
45.0esr-1 in March 2016. Transitional packages to ease upgrades were
provided in the wheezy, jessie, stretch and buster releases. The
transitional packages have been removed finally before the bullseye
release in August 2021.
After regular security support for buster ended in August 2022 and LTS
support ended in June 2024, I'm closing the remaining bug reports now.

Andreas