Functional procedure:
uninstall icloud from firefox
uninstall icloud from windows (logout required)
restart pc
install the new icloud for windows
restart pc
Sign in to iCloud
select in icloud for windows bookmarks firefox backup
agree to install icloud for firefox
That is all.

Sa y'est Icloud a sa mise à jour enfin! sa fonctionne dans toutes les versions!

Not working on Windows10!
同步是单向的：
iCloud→Firefox (√)
iCloud←Firefox (×)
无法同步回去

终于tmd能同步了..

It's happening! HOORAY! I've been using 32-bit Firefox for way too long. This addon is important to me. I've been waiting and waiting for this to be 64-bit compatible through an update to finally jump the ship to 64-bit Firefox, it's gotten depressing... until today! Windows 10 here running 64-bit Firefox. This addon works. Apparently just updating this addon doesn't seem to work, then I've uninstall this addon and the actual iCloud software off from my PC then reinstall both and they work.

works great