Firefox Multi-account Containers - Sign in to two different accounts on the same site

Temporary Containers - Firefox Add-on that lets you open automatically managed disposable containers

Lightbeam for Firefox - Mozilla is the not-for-profit behind the lightning fast Firefox browser. We put people over profit to give everyone more power online.

island - a free and opensource alternative to parellel space

Multilogin - Connect to websites as a multitude of different users

Incogniton - Incogniton is the tool for anyone who wants to say anonymous online. Using websites with multiple accounts at the same time is made easy. It protects users from being detected via (unauthorized) browser fingerprinting. 100% privacy and anonymity.