It is possible, but it's messy in terms of which one is the default (clicking on links outside of the browser), taskbar/launcher buttons (OS- and DE-dependent, some configurations have issues e.g. with pinning) etc. I think there was something else - minor and just slightly inconvenient - but I forgot.
Compared to ProfileManager, Container Tabs are absolutely awesome. Completely different experience, smooth and nice.
ProfileManager is a bit awkward indeed. You can pass `-p primaryprofile` and `-p secondaryprofile -no-remote` to make sure that links from OS are not opened in the 2nd instance, but then, when you close Firefox and click an external link, it will open Firefox with the last opened profile, which might be `secondaryprofile`.
The solution for me was to use one Firefox installation per profile: primary = stable, secondary = nightly.
Container tabs are way nicer than this, but since I open lots of tabs, I still like the distinction between stable and nightly (stable = regular browsing, nightly = FB).
Compared to ProfileManager, Container Tabs are absolutely awesome. Completely different experience, smooth and nice.