Was that mentioned in one of the replies? I don't see any specific libraries mentioned in the linked message, or the other replies I searched through.

edit: Scratch that, it was in this reply: https://mail.mozilla.org/pipermail/tb-planning/2017-March/00... . "I'd suggest using TypeScript, Mithril.js, Tachyons, and Node.js for the base technologies -- probably with Mocha/Chai for testing. Later we can make a more stand-alone desktop version with Electron or such. We could use socket.io to push notifications for changes in real or virtual folders and then use Mithril.js (a vdom library) to quickly rerender changed data the client UI requests (or is pushed)."

Worth noting that reply looks more like a wishlist of stuff than a meaningful viable proposal.