Replacement for ability to "send tab to remote device" as removed from Firefox 29 Firefox 29 removed the ability to "send tab to remote device".
Namely, you could send a share intent from any other app (select "share", select "Firefox", select any other Firefox Sync device you want to send the tab to) and a few seconds to minutes later it would appear e.g. in your desktop Firefox as a new tab.

That was my main use of Firefox on Android. How do I get exactly that back?

That is, I want send tabs from an Android share intent to Firefox on the desktop! Not the other way (I do have FoxToPhone, but hardly ever use it).
It should be a one-click operation, I don't want to copy/paste URLs, I don't even need a target device selection (although I would be okay with the two-click operation required in Firefox 28), because I always want to send to the same device.

This should work even when the Android device is offline (plane), in which case the tabs should simply be queued and sent when the network comes back (that's exactly what Firefox 28 did).

The use case is everything I spot on mobile (from emails, interesting stuff while browsing, reading articles on Pocket, etc.) but that maybe doesn't work on mobile (Flash), requires more screen estate or just requires some kind of action from me, which I don't want to perform on mobile.
Sorry for the trouble this is causing you. We initially removed the feature because we didn't know how well it was being used and the UX around it seemed confusing. We intended to improve the feature and release it again soon.

That said, with feedback like yours, and many others, we are reconsidering the decision and will try to get a version of the feature back in Firefox as soon as possible.

You can track some of the discussion here:

https://bugzilla.mozilla.org/show_bug.cgi?id=1002628