Firefox does support multiple isolated instances, just without a prominent UI. You can use the Profile Manager (https://support.mozilla.org/en-US/kb/profile-manager-create-...) or command-line flags to launch and manage separate profiles.

The UX around multi-account containers is still being worked on, but the contextualIdentities WebExtension API (https://developer.mozilla.org/en-US/Add-ons/WebExtensions/AP...) allows add-ons to interact with containers, so a third party developer could create exactly the experience you've requested.