Try the Firefox version from the official Mozilla server if you currently use a version from the repositories of your Linux distribution.

You can find the full version of the current Firefox release (56.0.2) in all languages and all operating systems here:

• https://www.mozilla.org/en-US/firefox/all/
• https://www.mozilla.org/en-US/firefox/56.0.2/releasenotes/

I downloaaded Firefox 56.0-2 into a separate folder and executed it from there but the problem is still the same. This is what is installed: firefox-56.0-5.fc26.x86_64

Anything showing up in the Browser Console when you click Sync?

• "3-bar" menu button or Tools -> Web Developer
• https://developer.mozilla.org/en/Tools/Browser_Console