Since these days I now use Firefox as my main browser (partly to give the finger to Google for the data thieving Chrome browser), I need to have a look at how to access all my Gmail accounts in the same browser without having to log in and out of whichever account I am using at a particular time. The same goes for, say, WordPress (I now have four WordPress blogs and each has a different account associated with it), Instagram and some other services. At the moment one workaround is to use several different browsers at the same time. For example I can use Firefox Developer, regular Firefox, Opera and Chrome.
Mozilla provide the Multi Account Containers extension to help you set up the capability to handle multiples of the same website account (for example, Google accounts) in the same browser window. You can set up a container group (for example Home, Work) and then add individual tabs to that group. Each group can operate independently of each other.
As good as this is, I am still considering an alternative of setting up multiple Firefox user profiles and starting a different Firefox session on a different user profile. This would still require the container groups to be used, but each group type would be unique to that user profile and there would probably only need to be one container group type per user profile.
I have been thinking about this for months (MACs have been available for a while and were first installed on Firefox some time ago for me) but now perhaps is the time I will start testing in earnest to see if I can make these ideas work.
The multi window thing addresses mainly that I don’t want a lot of tabs open in a window as would be the case with MACs all opening in one window. Being able to label the window and/or colour its title bar independently of other windows would also be a help. However, it could just be a lot simpler to classify everything using the MAC container categories sufficiently descriptively bridging what the different accounts are used for. I may still keep Firefox standard and Developer both in use with MACs.