There were no further details provided the plans are likely in the earliest stages of development.
Unfortunately, Firefox Send never returned, as it wasshut down in September 2020along with Firefox Notes.
However, the service was open-source, so forks likesend.vis.eeremain online.
Mozilla reviving Firefox Send for use in Thunderbird makes sense.
However, the Thunderbird team would have to do the work that Mozilla couldn’t do in 2020.
