Please check if there is any extension that is blocking the Firefox container option. Moreover, also check that

privacy.userContext.enabled

is set to true.

You can try to hold the left mouse button on the new tab plus button to see if that opens the container menu.

See also these prefs on the about:config page:

• privacy.userContext.ui.enabled
• privacy.userContext.newTabContainerOnLeftClick.enabled

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox