I downloaded and installed 81.0.2 (64-bit). The synchronisation works now. Thank-you.

I put the information from Help - About. Is there some other way to get the version number?

I also have this problem. I have just set up a new PC with Firefox Quantum 68.11.0esr (64-bit) and added Eversync extension. When I click the Eversync icon (top-right of the browser window, it shows 'Sync Bookmarks is not active, a 'Locked' button and 'Initializing  0/0'. I have also installed Google Chrome 86.0.4240.75 (Official Build) (64-bit) and the Eversync extension. On Chrome synchronisation works fine. On my old PC I had Firefox Quantum 68.7 (64-bit) and Eversync works fine.