(Copying reply from the bug)
For Firefox desktop, see this discussion: https://discourse.mozilla.org/t/about-requesting-a-new-locale-for-firefox/86515
Before starting to work on that, you should familiarize yourself with Firefox/Focus for Android and their release cycles (they are now available in Pontoon).
For the future, as a manager you request projects directly from Pontoon if they’re available (Firefox Desktop isn’t, and the explanation is in the link above): https://mozilla-l10n.github.io/localizer-documentation/tools/pontoon/teams_projects.html#requesting-a-project
P.S. several folks are out for the holidays, so expect slower replies until January 