installed in Firefox but not syncing

I installed and have a Sync Now item under Tools, but nothing happens when I click it.  There is no message or any delay at all.  When I open Add-ons > Extensions and click the EverSync preferences button an EverSync page is momentarily displays (less and a second) and then the top level Add-ons page is shown again.  I tried uninstall and re-install, but no change.  How do I fix this?