Do you use the NoScript addon or anything similar. If you do, disable it, while you setup Sync.

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the add-ons is causing the problem (switch to the DEFAULT theme: Tools > Add-ons > Themes).

• Don't make any changes on the Safe mode start window.

See:

• Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

If the problem persists then try "Reset all user preferences to Firefox defaults" on the Safe mode start window.