This feature appears and disappears randomly and it has me demented. Much of the time it works, and I am definitely signed in to FF Sync on desktop and mobile at all times. But that Firefox icon just vanishes for weeks at a time from my mobile browser. I even tried sending a tab from desktop to mobile recently (to troubleshoot) and it arrived over 24 hours later as a notification. Still no way to send tabs back to the desktop at the moment though, and all I can do is hope it reappears some day.

I see it in the menu on top right, and on android share dialog. Always worked in general, though with frustratingly long delays at times.