There is a desktop app. I am hopeful with EU cracking down on Apple will eventually result in Briar becoming available on that platform. I am working on an idea to connect people on Briar for use of the private groups and forums so you might check back with me in a few weeks to see where that’s at.
IMO, organizations like Mozilla should have their own Mastodon servers with closed sign-ups so that employees and volunteers can have a space to publicly discuss and promote their work in the organization. This should especially true of media organizations. In this case, I am almost sure the guy who was running it just got too busy or lost interest.