Simple: I don't believe you. Grab this copy of Firefox from 2022, https://www.firefox.com/en-US/firefox/96.0/releasenotes/, and run it on a modern distribution in 2026. If you fail, my point is made.
The page didn't include a download link, so I found it here: https://ftp.mozilla.org/pub/firefox/releases/96.0/linux-x86_...
And yes, it runs perfectly fine on Debian 13.
You aren't going to convince him because he doesn't know what the problem is.
Why specifically FF? Software without TLS does run just fine 5 years later on 5 year newer distros.
The page didn't include a download link, so I found it here: https://ftp.mozilla.org/pub/firefox/releases/96.0/linux-x86_...
And yes, it runs perfectly fine on Debian 13.