What's wrong with profiles in Firefox? I have 3 different Firefox profiles that I use, just for myself. (1 for general usage, 1 for banking, and one for a particular email account.) From my point of view, the profile system works fine. I don't know what else I'd even ask for to improve it.
And even still, I wouldn't expect most people would even want multiple Firefox accounts for a single computer login anyway; which is why the Firefox profile selector is disabled by default. (Tab-containers are a bit different, but related. I can certainly imagine a lot of people befit from those.)