The best open source alternative to Multilogin is Firefox Multi-account Containers, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 10 alternatives to Multilogin and three of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to Multilogin are Temporary Containers (Free, Open Source) and Containerise (Free, Open Source).
The Firefox Multi-Account Containers extension lets you carve out a separate box for each of your online lives – no more opening a different browser just to check your work email! Here is a quick video showing you how it works.
Easily open disposable containers that isolate attached data and are deleted after usage: Fully automatic, based on navigation-target, for certain websites, with configured mouse clicks on links or just by using the toolbar icon.
Firefox extension to automatically open websites in a container. Map multiple domains and subdomain to a container with simple CSV based config. Once mapped, this extension will always open the site in a container. This behavior is disabled in private mode.