Firefox on Debian stable is so old that websites yell at you to upgrade to a newer browser. And last time I tried installing Debian testing (or was it debian unstable?), the installer shat itself trying to make the bootloader. After I got it to boot, apt refused to work because of a missing symlink to busybox. Why on earth do they even need busybox if the base install already comes with full gnu coreutils? I remember Debian as the distro that Just Wroks(TM), when did it all go so wrong? Is anyone else here having similar issues, or am I doing something wrong?
It got better, that's what happened. You're using Firefox ESR, it's not unsafe.
Mozilla also provides a Deb repo for Debian and its derivatives: https://support.mozilla.org/en-US/kb/install-firefox-linux#w_install-firefox-deb-package-for-debian-based-distributions
Yeah and Flatpaks also exist.
Flatpaks are probably the best generic solution for using an LTS release like Debian Stable on a desktop system. You get the best of both worlds: up to date desktop packages and a stable base.