To my knowledge, per jdm-contrib/jdm#181, this addon does not work currently, and will not work due to the way Firefox has changed their handling of extensions. With that said, what we have present in the Chrome extension repo may actually work with little-to-no changes for Firefox. (research needed)
The main justdelete.me repository has been abandoned and we're trying to keep it alive on a detached fork here, would you be interested in transferring the repo to the jdm-contrib organization?
If we do we can assure it is kept up-to-date and, since we have gathered some people on the org, not let the project get stale.