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:
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:
Could you please update cbindgen in byzantium accordingly?