Broken, and has been for years on Windows 8.1 and 10. It will synchronize once and then completely breaks and never updates again (iCloud forgets the addon is installed) Have to wipe all bookmarks, delete the addon, and start all over in order to get it to synchronize one more time. Not worth the effort.

Edit: Update 2022 - Addon still constantly breaking. Extremely lame. Seems to stop working if I don't synchronize every week. Really hard to safely reset without wiping out all bookmarks or duplicating everything.