Starting a profile is difficult, and also switching between windows, since the OS sees profiles as different apps. It's probably not hard to fix, but Mozilla has had other priorities.
I don't know why they don't add this somewhere in the menus and I don't know why you can't bookmark about:profiles, but this works well for me as it is.
Add-on: https://addons.mozilla.org/en-US/firefox/addon/profile-switc...
Source: https://github.com/null-dev/firefox-profile-switcher
- Junction (Linux browser picker): https://github.com/sonnyp/Junction
- Finicky (macOS browser rule setter): https://github.com/johnste/finicky and Browserosaurus (macOS browser picker): https://github.com/will-stone/browserosaurus
- Hurl (Windows browser picker): https://github.com/U-C-S/Hurl
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.
That said, containers are a handy tool for people who want to access multiple accounts without the trouble of setting up individual profiles. Keep in mind, if you're only trying to access a personal and work Google account you probably don't want to spend time setting up a separate profile for each then keeping the settings synchronized.