Forum response - Containers add-ons may reset

Containers add-ons may reset to their default state. If add-ons that use the Containers functionality, like Multi-Account Containers and Facebook Container, were disabled as part of this problem, any lost site data for containers, as well as deleted custom containers you may have configured, will not be recovered by this new release.

Data that may be lost for Container add-ons include configuration data for which containers to enable or disable and container names and icons. Locally-stored data for the container (cookies and permissions) are also affected. See Bug 1549204.

See Multi-Account Containers to set them up again.