The device your currently on will not show. If it's not the current device, you should go log into the device you're looking for. I'm currently on a mac and it' not listed in the send to. see screenshot

I went to create screen shots, and the missing entry for Device R is now visible again on Device M. Not sure why.

To recap, Device R was not shown in the list of "Send Tab To Device" on Device M. On Device R, the entry for Device M was never missing.

Other than manually Syncing Firefox (which I did), what else can be done to bring back a missing device? Waiting a day is not ideal if you're in the middle of working on something.

What is odd is that I use Device M and Device R every day, and there are devices on the list that I have not used for years. Why would an active device drop off when many "hibernating" devices remain on the list?

I went ahead and attached the screen shot from Device R in case there is any question about what we're talking about.

Is that missing device currently active and connected to Sync ? What kind of device is this (desktop or mobile) ?

Syncing tabs should work when sync is working.

Note that send tab to device is using a push operation and the OS on the device would have to support this. Note that you can disconnect devices that you no longer use to cleanup the list.