Found while reviewing D167729 (bug 1811114). In the deb repackage task[1], we have a bunch of warnings like this one:

[task 2023-02-16T21:07:47.888Z] 21:07:47     INFO -  dpkg-shlibdeps: warning: couldn't find library libnss3.so needed by debian/firefox-nightly-try/usr/lib/firefox-nightly-try/gmp-clearkey/0.1/libclearkey.so (ELF format: 'elf64-x86-64'; RPATH: '')

(See attached logs for the exhaustive list)

:jcristau shared his knowledge with me. At startup, Gecko modifies the environment variable LD_LIBRARY_PATH to inject the folder where the firefox binary is. This means these warnings should be harmless. We should be able to tell dpkg-shlibdeps about the location of these libraries by editing rules[3] to include something like:

override_dh_shlibdeps:
	dh_shlibdeps -ldebian/${DEB_PKG_NAME}/usr/lib/${DEB_PKG_NAME}

This way we should have no more warnings like this and we should be able to spot other types of warnings more easily.

[1] https://firefox-ci-tc.services.mozilla.com/tasks/dNsyVoorTmmkWI0eFREvdA/runs/0/logs/public/logs/live_backing.log#L478
[2] https://searchfox.org/mozilla-central/rev/d85572c1963f72e8bef2787d900e0a8ffd8e6728/ipc/glue/GeckoChildProcessHost.cpp#1126-1137
[3] https://searchfox.org/mozilla-central/rev/d85572c1963f72e8bef2787d900e0a8ffd8e6728/browser/installer/linux/debian/rules#7

Looking into it...