All devices have the same status and that means that the data from each device is merged to other connected devices.

1. Sync doesn't work with iOS devices - IOW, not with the iPhone. Mozilla is working on a browser for iOS devices, but I haven't heard about a possible release date yet. I might be called "Firefox" when it comes out, but in name only. From what I have read Mozilla has to create an all new browser.
2. The Sync server is the "hub" and all connected devices are the "spokes". Each device communicates with the "hub", not to any other device directly.
3. Overall, depending upon how much data you have in Firefox, the initial Sync process for each device can take a bit of time. And if you attempt to do the initial Sync for all devices at the same time, I have found out the hard way that it might take a few days for everything to be fully synchronized.

My menu button (the 3 stripes) doesn't offer an option to Sign in to sync and my 2 PCs on the same home network do not appear to Sync. Does this take long?

Try to enter about:accounts in the address bar.

If you use a recent version of Firefox (30 or higher), you should have a button Sign in to Sync in the menu like in the attached screenshot.

Sync isn't peer to peer across a local network, the Sync server is in the "middle" of the Sync process.

Solved my issue. Most of the problem was my own fault so I'll let it go there. Thanks to all.

jbacinti

Thanks to cor-el we now know that the initial sync merges all data. But the point is that a new user needs to know that before signing up. Needs to know that because one device has little data on it the data on another device will not be removed, etc. Why can't the documentation just give a simple account of what will happen. The superficial statements like "will keep all your devices synchronised" don't cover it.