Seems odd that it doesn't work on Firefox in that case, as Jitsi meet works on Firefox. I wonder what they've done / added that lead to dropping support for Firefox.

Edit: I guess it's strategy, pushing more people to use Brave browser, rather than an actual limitation of the software.

It literally just checks whether navigator.brave.isBrave(). The following Greasemonkey script is sufficient to fool it:

    // ==UserScript==
    // @name        Very brave
    // @match       https://together.brave.com/*
    // @grant       none
    // ==/UserScript==
    navigator.brave = { isBrave: () => true };