In case screensharing isn't working in Jitsi Meet with Firefox 74: This is a bug and should be fixed with https://github.com/jitsi/jitsi-meet/pull/5180.

You can apply that change easily to an installed instance:
https://guides.lw1.at/books/how-to-install-jitsi-meet-on-debian-or-ubuntu/page/some-more-things