when i had the error i also had the xmarks add-on installed but disabled.

anyway i had to refresh my windows so i reinstalled the latest firefox and eversync and it works

of course, here they are.

if i uninstall the add-on then restart firefox and reinstall it  the add on will work for a while but after a restart it will stop again

i'm using the latest version of EverSync, which was released 2 months ago. I still get the error: LOCKED

I see the same error in Firefox 60