Why the fuck is it weird for a client to run inside a runtime environment with well-defined library versions, when that client has to ship that runtime environment anyways to ensure the games can actually run on a broad range of systems?

Oh, sure, it's so much better if the client relies on system libraries instead. Yes please, I like incompatibilities and issues with debugging. So much better than loading a different set of libraries. Stupid Valve!

Funnily enough, people running in Ubuntu do get Firefox in a container by default IIRC as it's delivered as a snap

I run most of my software in containers. Firefox is in a flatpak. My terminal shells are all containers using distrobox. My homelab services are all containers. My few VMs (i run a few vituralized rke2 clusters, sometimes a test version of my baremetal harvester cluster, and test versions of my desktops)? Also running in containers. My desktop OSs are also containers (ublue, SteamOS, and SUSE Elemental).

The future is now old man! :p

But honestly linux namespaces and overlay filesystems are the bees knees. Create reusable layers of filesystems, use just the ones needed for a given app/service. Expose just what a service or app needs to for a given function. You end up with an extemly portable, and consistent system that has cleaner seperations of concerns. For basically free. From an app dev perspective you remove a whole matrix of supported configurations to worry about (distro/version/packages installed/etc).

That's not really Valve's fault: Tell the other companies to suck less! All the other services are fucking terrible by comparison, GOG is the only one that comes close.