You can check the about:sync-log page via the location/address bar to see if there are recent error logs available. You can upload a recent log if the log is too big to put in a reply.

• http://pastebin.mozilla.org

Make sure you allow cookies for the accounts.firefox.com domain if you aren't allowing cookies generally or use extensions that deal with cookies.

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is causing the problem.

• Switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Appearance
• Do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

Here is the error log via pastebin: https://pastebin.mozilla.org/8958653

There is a problem with the log in.

1483576113598   Sync.Service       INFO   Starting sync at 2017-01-04 16:28:33
1483576113598   Sync.Service       DEBUG  In sync: should login.
1483576113599   Sync.Service       DEBUG  Exception: Could not acquire lock. Label: "service.js: login".
1483576113599   Sync.Service       INFO   Cannot start sync: already syncing?
1483576113599   Sync.Service       DEBUG  Not syncing: login returned false.

1483576177305   Sync.Status        DEBUG   Status.login: success.login => error.login.reason.network
1483576177305   Sync.Status        DEBUG   Status.service: success.status_ok => error.login.failed

Make sure that your security software isn't blocking access to the sync server.

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is causing the problem.

• Switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Appearance
• Do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

Boot the computer in Windows Safe Mode with network support (press F8 on the boot screen or hold down the Shift key) to see if that has effect.

• http://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/
• http://www.7tutorials.com/4-ways-boot-safe-mode-windows-10

Unfortunately, it did not work with Windows Safe Mode and Firefox Safe Mode.

Here is the error log from that attempt: https://pastebin.mozilla.org/8959023

In case of issues with Sync you can try this:

Disconnect from Sync and Reconnect to Sync again. You can disconnect from Sync by clicking the e-mail address in the "3-bar" Firefox menu button drop-down list or go to "Options/Preferences > Sync".

Delete the signedInUser.json file in the Firefox profile folder and delete the Weave folder (Sync was formerly named Weave).

Remove an existing chrome://FirefoxAccounts entry in the Firefox Password Manager.

• Options/Preferences -> Security: Logins: "Saved Logins" -> "Show Passwords"

If that isn't enough then delete the prefs.js files to reset all prefs and maybe also delete the compatibility.ini file to reinitialize the profile.

Delete and re-create the Firefox account or change the Sync password. Note that changing the Sync password will wipe all data stored on the Sync server.

You can use this button to go to the current Firefox profile folder:

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

Many hours later... Success. If you want to skip over the process, scroll down to "SOLUTION."

Note: I did all of the following with my anti-virus turned off, just to make sure that wasn't the source of the conflict.

I tried the following: - Disconnect and Reconnect - Delete signedInUser.json and Weave folder - Cleared Firefox Password Manager (there were only a few references, because I don't use it. chrome:FireforxAccounts was not present) - Delete prefs.js and compatibility.ini

After doing all of those things and still not finding success, then I uninstalled Firefox. Still didn't work. So I uninstalled Firefox again AND deleted the Mozilla folder in Roaming. Reinstalled...

After all of this, now I am left with a situation where it won't even let me sign in to Firefox Sync. I keep getting "Unexpected error." This is the same result when using Firefox in Sync mode AND after choosing "Refresh." I have tried many variations of steps...

I tried changing my password on a different device and then using that to login, but I'm still getting the "Unexpected error."

I tried deleting and recreating my account, but I'm still getting the "Unexpected error" on this computer only.

SOLUTION

I'm still confused about the exact dynamics, but here is what I did... Since I knew the firewall was disabled on my computer, I decided to check my router (Aus RT-N56U). There was a firewall there, so I disabled it. And... SUCCESS. Firefox would suddenly let me login to my account and sync.

Strangely, I re-enabled the firewall on my router and it continued to work. It seems that after connecting once under those circumstances has cleared some sort of roadblock.

If someone has a theory or explanation, then I'd love to hear it. Thanks!