I'm surprised to see they haven't added the close option in the Synced Tabs sidebar. The UX would be a lot better for users that need to manage multiple devices/tabs.This was a bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1917342. This bug also ...
The ability to close remote tabs was pushed to 131 (now on beta). You can find the control (an X on the rightmost part of the item) on the list of remote tabs in the account menu and under the application menu.
