Thank you! I'm looking into the ubuntu 18.04 issue and will hopefully be out with a fix soon.

EDIT: Documented in https://github.com/subdavis/Tusk/issues/146 and due to ibus/ibus bug

The pop-up can be accessed in a normal tab.

1. Navigate to about:addons in Firefox
2. Click preferences next to Tusk
3. In the address bar, change options.html to be popup.html

You can bookmark this page for quick access.