Lists existing identities, lets you create new tabs with an identity and remove all tabs from an identity. For more information on contextual identities: https://wiki.mozilla.org/Security/Contextual_Identity_Project/Containers
How to use the contextualIdentities API. Please note: you must have contextualIdentities enabled. You can do that by going to about:config and setting the privacy.userContext.enabled preference to true. If you are using web-ext you can do this by running:
web-ext run --pref privacy.userContext.enabled=true
Icon from: https://www.iconfinder.com/icons/290119/card_id_identification_identity_profile_icon#size=128, License: "Free for commercial use".