What you mean with "not working". Can I help?

There was an issue with containers but has been fixed successfully.

Please note as well web extensions are a very new in Firefox and there lot of (known?) issues, nothing to do with this extension.

E.g. Firefox starts and shows anything but not Nellitab:
https://discourse.mozilla.org/t/url-about-newtab-still-shows-default-and-not-custom-new-tab/27593/5

For such issue i can only offer to wait until it has been fixed by Mozilla.