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?
You are literally describing the idea of Debian. Yes, stable is old, but that is the whole purpose. You get (mostly) security updates only for a few years. No big updates, no surprises. Great for stuff like company PCs, servers, and other systems you want to just work™ with minimal admin work.
And testing is, well, for testing. Ironing out bugs and preparing the next stable. Although what you describes sounds more like unstable, the one where they explicitly say that they will break stuff to try out other stuff.
So, everything works as intended and advertised here. If you want a different approach to stability, I guess you will have to use a different distro, sorry.
I guess when you last tried it, it was at a time when a new stable came out, so testing was more or less equal to stable.
About the firefox: It ships Firefox ESR these days, meaning you get an older, less often updated tested firefox (with security updates, of course). Again, this is the whole point. Less updates, less admin work, more time to find and fix bugs. Remember the whole Quantum add-on mess, for example?
As others have said, you can install other versions of firefox (like the "normal" one) via flatpak, snap... nowadays. The same goes for other software, where you would need the newest and shiniest version sooner. I'm using debian on my work/uni laptop and a bunch of servers, and it works pretty well for me.
The last paragraph is vital. Grab a flatpak of any software you need to be more up to date. Flatpaks running on Debian are amazing. Current software running on a stable base.