chromium

Firefox uses profiles to manage personal data like logins. In order to create profiles besides the default one you can use the Profile Manager by opening "about:profiles" in the address bar. If you often find yourself switching between different profiles you might find the plugin Firefox Multi-Account Containers useful. It allows to color code tabs belonging to a profile to help telling them apart.

Chromium and Google Chrome also allow to use multiple profile in the same browser instance. A straightforward way to make use of that is the plugin SessionBox.