"Enable Container Tabs" option missing in Firefox 97 on Ubuntu 20.04

Question

On several systems, I have Firefox set up with container tabs, which I have enabled by going to

[menu] > Settings > General > Tabs > Enable Container Tabs (checked)

However, on my newest virtual machine running Ubuntu 20.04, when I click the same places, I cannot see any option for "Enable Container Tabs". The option simply isn't here, and searching in settings for "container" doesn't return any results. What is wrong here?

===(More details below)===

PS: Yes, I can go to about:preferences#containers and it seems to work, but no I cannot actually open a tab in a container. (Just as I would not be able to open a tab in a container if I unchecked "Enable Container Tabs" on a system that has the option.)

PS: Does "Enable Container Tabs" have any equivalent setting in about:config?

PS: I'm trying to avoid installing any extensions (such as the multi-account containers extension) and I was able to do what I'm asking on several other systems. I don't understand how "Firefox 97" on one system can have the option and on another system it doesn't have it.

Answer 1

I found an advanced way to enable container tabs without any extension: under about:config, set privacy.userContext.enabled to true and privacy.userContext.ui.enabled to true.

Then press Alt, F, B to see the list of containers and click one to open a new tab in it.

...and after that, it starts showing "Enable Container Tabs" in the general settings under Tabs. So that could explain why it was showing the option in settings on the systems where it was enabled earlier --- the option is only shown if it's already enabled (?)