https://github.com/hultmann/multifox/releases/tag/3.2.3 Multifox is an extension that allows Firefox to connect to websites using different user names. Simultaneously!

Have you considered using two different Windows accounts? Firefox will create completely different settings folders for each user. But it is more work going back and forth between different Windows users than sharing one user. (Also, you might need to exit Firefox when switching users, but I haven't tested that myself.)

The Multifox extension has a long history, but upcoming changes in Firefox 57 will make it obsolete. A new technology that is still being finalized is Containers, which lets you open links into different groups of tabs that have their own cookies.

You can learn more about Containers and consider testing them here:

https://testpilot.firefox.com/experiments/containers

Although Containers has some preset categories, you'll probably want to create new ones with more relevant names.

Hope this helps.