In bug#1279297, we added the ability to populate the "Customize" window under the hamburger menu with the "Open Container Tab" button without having to restart FX once the container feature has been enabled. However, it looks like there's some cases where this doesn't work as intended. I've added two examples with STR below: STR: (example: https://youtu.be/QjSJktRzoNc) * launch the latest version of m-a (or any channel that doesn't have containers enabled by default) * before enabling containers, view the "Customize" window via the "Hamburger Menu" * enable containers via the privacy.userContext.enabled;true pref under about:config If you take a look under "Customize", you'll notice that the "Open Container Tab" button won't be visible. Once the browser is restarted, it will correctly appear. This happens only when you visit the "Customize" window before enabling containers. When you enable containers without viewing the "Customize" window under the hamburger menu, the icon will be visible as expected. The other issue is that the icon doesn't disappear correctly from the "Customized" window once the container feature has been disabled. If the user doesn't restart their browser, they'll be able to use the "Open Container Tab" button which might make it look like they're using containers even though the feature has been disabled and nothing is being compartmentalized. STR: (example: https://youtu.be/5ogLvPj6RjA) * open fx and ensure that the container feature is enabled * ensure that the "Open Container Tab" button is visible in the "Customize" window under the "Hamburger Menu" * disable the container feature via privacy.userContext.enabled;false under about:config You'll notice that the "Open Container Tab" button will still be visible and usable under.

Bulk change per https://bugzilla.mozilla.org/show_bug.cgi?id=1401020