Firefox Sync works but the "Send to Device" contextual menu item hidden by m-c-f. Perhaps a pref under services.sync.* like services.sync.engine.tabs could be used in our autoconfig.js file to (un)hide depending on Firefox Sync status.
I have tried a different preference identity.fxaccounts.toolbar.accessed in my commit to urlbar.css last night prior to reading Danny's comment, see !64 (closed)
The preference I picked leads to the following behaviour:
upon loging into your Firefox Account, the menu stays hidden in portrait (as some people may just want to pick up on desktop where they left on mobile, or just sync addons or whatever)
after changing the value to true in about:config (doing so by once tapping the unhidden accounts icon in landscape) it shows up.
The menu behaves okay in my testing on Phosh, GNOME Mobile and Sxmo.
We could also unhide the accounts in app menu (which in turn would also require a height-adjustment to avoid a scrollbar), which may be more glitchy (especially as popups tend to behave slightly differently on the different UIs), but would save space in the urlbar.
When going the appMenu route, #45 could also be tackled while doing so (to just do all the boring testing across UIs once).