Your comments

Nevertheless, it is still locked, even after re-installation of the plugin and clearing all the browser caches.

Originally reported from Fedora 27, but I installed Ubuntu 17.10 (incl. updates) which suffers the same issue on Firefox Quantum 57.0.1. The Chrome browser is not affected by the bug. Interestingly, the Sync feature has been working initially on FF, but stopped working after a while. The only thing I did was somehow rearrange my bookmarks, moved some of them, deleted them. Maybe the basic CRUD operations are responsible for the lockup.