The -ProfileManager flag to Firefox gives a startup window to manage, create, and select profiles (and has existed forever).
You can create shortcuts (or shell scripts) by hand with --no-remote -p ProfileName to auto-open specific sites in specific profiles. It's certainly less convenient than PWA installs, but is an old tool with all sorts of old ways to automate it.
Multi-account Container is far easier than all that though and does reduce the number of windows to manage in your OS taskbars. Obviously you need to be careful with which extensions you install that they are safe for every container you want to use, but that's a given and a good idea in general no matter how many profiles you use: keep extensions to the bare minimum you are comfortable with. Vet their permissions manifests and keep additional permissions you grant them to a minimum.
You can create shortcuts (or shell scripts) by hand with --no-remote -p ProfileName to auto-open specific sites in specific profiles. It's certainly less convenient than PWA installs, but is an old tool with all sorts of old ways to automate it.
Multi-account Container is far easier than all that though and does reduce the number of windows to manage in your OS taskbars. Obviously you need to be careful with which extensions you install that they are safe for every container you want to use, but that's a given and a good idea in general no matter how many profiles you use: keep extensions to the bare minimum you are comfortable with. Vet their permissions manifests and keep additional permissions you grant them to a minimum.