Hi. At 1st Enter the setup code on your computer to connect your other device to your Sync account then Click the Home button (or type about:home in the location bar) to bring up the default Firefox home page. Then At the bottom of the home page, click the Sync link to open the Sync settings window. Then click the Pair a device link and the Firefox Sync Setup window will open.and Enter the twelve character code generated on your other device and click Next. When the code is accepted you will see a Device Connected message and your device will immediately begin syncing. Click Finish to close the setup window.i think i will help u :-)

I do that, and sync working fine, but 2 weeks ago I start to receive that error

https://support.mozilla.org/en-US/kb/firefox-sync-take-your-bookmarks-and-tabs-with-you i think it will help u

there are also some server issues, maybe you're affected by that: https://services.mozilla.com/status/