You're talking about Thunderbird, a project they basically abandoned to the community. Thunderbird survives in spite of Mozilla, not because of it. Meanwhile their main product Firefox is still bleeding users down into the single digit percentages while receiving half a billion a year from Google. It takes a lot of skill to run such a company so deep into the ground.
Perhaps a controversial opinion here, but the usefulness of reversibility is vastly overrated. It's not a game changer, just a tiny first-world luxury that's nice to have, but it does it by introducing a bunch of unnecessary complexity that I'd rather avoid. Not worth the trade off IMO. I can count on one hand the number of minutes USB-C has saved me by being reversible and I honestly don't care