No screen sharing on Firefox with Wayland

[Mossroy]

On Ubuntu 20.04.2 or 20.10, if I switch to a Wayland session, and open Firefox (currently 85.0.1) on a site using some screen sharing, Firefox does not list the available windows but only a line "Use operating system settings".
After choosing this only option, nothing happens and my screen is not shared.

When using the default Xorg session, Firefox lists the available windows and allows to share them.

Steps to reproduce :
- open a Wayland session
- open Firefox on https://meet.jit.si/
- start a conference and join it
- try to share your screen (icon of a screen at the bottom left)

Expected behavior : Firefox should ask me which window to share (like on Xorg)

Actual behavior : Firefox does not list any window, but only a "Use operating system settings" line, and selecting it does nothing

NB : installing and starting pipewire (as suggested in http://jgrulich.cz/2018/07/04/how-to-enable-and-use-screen-sharing-on-wayland/) does not help
NB2 : the behavior is the same on other sites, like Google Meet

[Mossroy (mossroy)]

I tested the same on Chromium 88 (on Ubuntu 20.10) : it does not allow to choose a window other than the one of Chromium (or one of its tabs), but it works in this case.
It allows to share the entire screen, but it broadcasts a black screen instead of its actual content

[Ubuntu Foundations Team Bug Bot (crichton)]

Thank you for taking the time to report this bug and helping to make Ubuntu better. It seems that your bug report is not filed about a specific source package though, rather it is just filed against Ubuntu in general. It is important that bug reports be filed about source packages so that people interested in the package can find the bugs about it. You can find some hints about determining what package your bug might be about at https://wiki.ubuntu.com/Bugs/FindRightPackage. You might also ask for help in the #ubuntu-bugs irc channel on Freenode.

To change the source package that this bug is filed about visit https://bugs.launchpad.net/ubuntu/+bug/1915802/+editstatus and add the package name in the text box next to the word Package.

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[Olivier Tilloy (osomon)]

The firefox side of things should be covered, but it's pending a mutter update in hirsute (bug #1730612).
It is highly unlikely that this will be backported to focal or groovy though.

I'm marking this bug as a duplicate of bug #1730612, if it still doesn't work after the mutter fix lands in hirsute, please de-duplicate. Thanks!