While desktop sharing using Chromium or Brave browsers offers 3 possibilities (screen(s), window but list none, browser tabs), with FF 74.0 (64 bits) Linux/Ubuntu desktop sharing is possible for the whole screen(s) only. No window selection nor FF tab.
This is not blocking, at least it’s possible to share something, but is that normal? If not, what could be done to be able to share a window?
In the meantime, with a dual screen, you must first disable one on them before sharing the entire screen.