So I downloaded a similar build, 73.0-1 for Arch Linux arm64 and replaced /usr/lib/firefox on my Debian sid installation with the respective contents of that build. And it works. So my guess is that it's something with the Debian build.
The build I tried is "firefox-73.0-1-aarch64.pkg.tar.xz" from http://tardis.tiny-vps.com/aarm/packages/f/firefox/
