Oh that's interesting, I didn't know that about Firefox sync. How does it work? You connect, once for all (with a login/pwd?) 2 browsers (e.g. phone Firefox and computer Firefox)?
Then you go to send.firefox.com, on phone, and you upload a file. And then how do I get the tab with the right URL on the PC? I guess I open Firefox, and then how can I see "tabs opened on my phone"? (Would you have a screenshot?)