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It's odd that the error in the logs seems to be no user name (error.login.reason.no_username).

Can you sign in on the website, or same issue: https://accounts.firefox.com/signin?service=sync

It's even more weird than that. I have a work laptop running windows 7 and can sync using my work network. When I bring it home and use my home network I get the unexpected error when I sync or try to set up a new account.

I cannot connect to firefox sync at home using my smartphone but I can sync my bookmarks at work. I can only think it has something to do with my home internet or wireless router.....makes no sense to me.

Ever heard of a problem like that?

I think this is the second or third thread I've seen where form submission behaves differently on different networks in Firefox 48. For example: https://support.mozilla.org/questions/1133493

For others who may want to try this.....it took me a while to figure it out but it was my home network. I simply unplugged the ISP moden for 2 mins and plugged it back it.

My work laptop was syncing at work but not at home. At first I thought it was windows 10 but it was the modem

I had the same problem. I simply restarted Firefox and it worked.