It's per window and it remembers the last profile used. I find it hard to manage as other apps will open links on the active window, which might be in the incorrect profile I want to use at that time.
I like this new feature in Firefox. OS-level profiles being the slowest method, Chrome profiles being faster but with this annoyances (for me)... Firefox new container tabs look like a more lightweight/faster method for context separation.
Yeah, this is actually distinctly different from Chrome's profiles. Firefox does already have an equivalent to Chrome's profiles [0], so it wouldn't make a whole lot of sense to implement this, if it wasn't different.