There's no "would be" about it, this is just an actual feature of Firefox
https://support.mozilla.org/en-US/kb/how-use-firefox-containers
edit: unless you mean specifically disposable ones for which there is an add-on
https://addons.mozilla.org/en-US/firefox/addon/temporary-containers/
Can't say I'm familiar, sorry
I'm pretty sure it would still be implemented at the engine level for mobile but AFAIK at least the regular mobile Firefox has nothing for it in the frontend