I've just loaded FF onto a new Macbook Pro (Monterey), and have also recently signed up for Firefox Sync - to import bookmarks, passwords, etc into this new installation. I'm perplexed at what I'm seeing on the new machine, though. I have the firewall on, all Sharing settings off, Bluetooth off, wifi off. No ethernet. The machine is hypothetically completely air-gapped.
But the new machine is displaying the Firefox sync icon (superimposed on the logo is a little picture of a laptop, hovering over it says "from laptop"). This icon appears when I open Firefox on the old machine. It's very unsettling. Nothing on the new machine indicates there should be any communication between FF instances.
Can anyone tell me how this would be happening? Is there some undisclosed p2p action going on between the machines based on physical proximity?
This appears to be an Apple OS feature called "Handoff". It works with multiple browsers, and offers to share browser tabs between your devices through iCloud, apparently. It also does clipboard sharing, and some other things.
It can be disabled through System Preferences > General > uncheck "Allow Handoff between this Mac and your iCloud devices". (For more: https://howchoo.com/macos/how-to-disable-chrome-tab-sharing-handoff-in-macos - article is about Chrome, but this seems to work with Firefox as well)
I have no explanation how your machine was still communicating with all the networking interfaces apparently turned off, though.