Regarding your last sentence, does that mean that in the future I could open a link in a 'tor browser' container? That's awesome if so.
Speaking as someone who is familiar with some parts of the Firefox architecture or concepts thereof, but not the source code itself: If you were to implement per-container pref overrides, theoretically yes. AFAIK, prefs are global right now. I don't know if it's feasible to implement this in Fx or if that will leak through to the chrome (process).
But it's an interesting idea! Would you mind filing a bug at bugzilla.mozilla.org?
For example, for First Party Isolation, we took the "origin attributes" feature that we built to support containers (user-specified tracking limitations) and reused it for isolation. In the containers case, origins get tagged with a user-specified label; with First Party Isolation, they get tagged with the top-level origin.
And to be clear, there's no "neutering" going on here. We're adding the full features that Tor Browser has, since the whole point of this exercise is to let Tor Browser user preference changes instead of patches. That means that the full capability of the Tor Browser features are in Firefox if users want to enable them.