https://addons.thunderbird.net/en-US/thunderbird/addon/filelink-provider-for-send/ this add on can be pointed at any send instance, so long as the instance isn't too diverged from the popular fork or Mozilla's original project.

The Thunderbird team has talked in the past about bringing Mozilla Send back as more of a feature for Thunderbird files embedded in emails, hence some of the work that's happening off and on by Mozilla themselves in the original project, some of which has been merged into the project this post is about.