I use Multi-Account Containers. I have Restore previous session enabled under Startup in settings. When I have container tabs open, and I close firefox and reopen it, it sometimes removes the container label from the tabs and tries to open them in No container mode. The easiest way to reproduce this problem is to open some tabs in a container, Go to MAC -> Select the container -> Hide This Container, then MAC -> Select the container -> Show this container, and immediately close firefox. When you open Firefox again, the tabs will have lost their container association.