Firefox ESR 78 has been EOL for months and has known unfixed security issues.
Alas, byzantium doesn't offer firefox-esr 91 (which is available in Debian since December) yet because the build fails:
https://software.pureos.net/package/builds/job/75163a5e-c53f-4503-99ba-a5ba15a01f4e
That is because it is missing (rust-)cbindgen >= 0.19 as build-dependency, which firefox-esr 91 needs.
Debian is offering cbindgen 0.20 via stable-proposed-updates because of this:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1001692
https://tracker.debian.org/pkg/rust-cbindgen
Could you please update cbindgen in byzantium accordingly?