Chrome has profiles. I use multiple profiles for this. You can easily have two or more profiles loaded and running at the same time, and each has its own cookie jar/login set. When I need to do it on a borrowed computer, then I just use incognito mode for the second profile.

I also use chrome profiles to separate my workspaces, so I don't have to load everything all the time and search through my tabs to find the one I want. I even sync them to different accounts.

Mobile is the only place I use multiple browsers, due to mobile browsers not supporting multiple windows, even though chrome has already had the functionality for it in the past (remember the chrome 'desktop' when chrome ran in metro mode? Alternatively, just have multiple entries in overview). On my tablet I have six browsers, which is probably more than I really need; two incognito browsers and four mobile browsers. Various combinations of add-ons (firefox) and not (chrome), incognito (focus and inbrowser) browsers, and browsers (chrome) with JavaScript disabled.

> (two separate Google accounts is basically unmanageable without separate browsers)

You can use Firefox's container tabs for this.