Firefox browser uses its own permission dialog to allow user to selectively pick which screen/application to share (See screen shot from https://support.mozilla.org/en-US/kb/screenshare-safety) However, unlike other browser (Chrome) or other video calling app (Zoom), Firefox only list the window/application name in its selector as oppose to showing the preview of each application. This make it very slow to pick which screen to share. Note that preview is available after the candidate window is select.
Additionally, since it is permission dialog. User can express an intent to "cancel" if he/she could find the target window to share. The only possible option is to share something or "block" sharing permission on this website which would require an explicit removal of the block (or restart the browser) before attempting to share again. This create an inconsistent video calling behavior experience when compare to other platforms