It is doable but is hidden behind special settings:/// etc sort of and switching between them also requires something similar.
Profiles allow for greater separation of browsing personas: I can have a different set of passwords, extensions, bookmarks per persona I have like Work, Personal, Focus etc...
And while Firefox does have profiles, its not a great user experience. Theres no quick switching, the UI to access it feels outdated, and in general as much as it pains me to say it, Google has done a great job at implementing it.
I'm sure Firefox will get there, but as of right now its a sub-par experience.