I have not tried any of this myself, but this is how I would do it. Make sure to have backups and be ready to roll back if something goes wrong.
First, add the Debian repo to your sources.list:
deb http://deb.debian.org/debian buster main
deb-src http://deb.debian.org/debian buster main
deb http://deb.debian.org/debian-security/ buster/updates main
deb-src http://deb.debian.org/debian-security/ buster/updates main
deb http://deb.debian.org/debian buster-updates main
deb-src http://deb.debian.org/debian buster-updates main
Then add the
arm64 architecture and update the package list
sudo dpkg --add-architecture arm64
sudo apt update
# don't upgrade
Note that it's not recommended to
upgrade while you have Debian repos as sources. I got weird issues after such an upgrade, and since quite a few Raspbian packages got replaced by Debian counterparts, I wasn't inclined to track down those issues.
Finally, install an
sudo apt install firefox:arm64
This will pull a lot of dependencies including the most basic ones like
libc, since the rest of your system is 32-bit. Then you should be able to run your 64-bit software alongside the rest of your 32-bit userland. If you have the same program in 32 and 64 bits, you'll have to find out where the 64-bit version is installed, my guess would be
/usr/bin/aarch64-linux-gnu/, or something along these lines.
Note: you may already know, but Ubuntu supports RPi and is known to run 64-bit binaries.