This add-on is built into Firefox / Tor. It works fine on my non-Whonix Tor. I just realized it does not work on Whonix and I don’t recall if I have tried to use it in Whonix in the past. It is enabled in the Preferences screen but there is no option to open a container in the context menu to be found anywhere.
Is this disabled for some reasons in Whonix? If not is there a way to reset this? Thanks.
Edit: I did go into about:config and confirmed the following were set as they should be;
privacy.userContext.enabled to true, privacy.userContext.ui.enabled to true and privacy.userContext.longPressBehavior to 2.