As many others have said, it doesn't work. Mozilla should probably just remove the extension until Apple decides to maintain it.

If you enable Firefox sync in the iCloud settings it directs you to download the extension which you have already downloaded. This is a straight up bug in iCloud but I don't have high hopes that it will get fixed given the generally abysmal quality of iCloud on Windows.

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.

رايع

It's great now being able to link your iphone to a regular PC now.

A strange thing. I'm Win10, and i installed icloud app in Microsoft Store, then checked the ff in setting. All things looked good, but it couldn't sync my bookmarks.
Then I tried to install the app for Win7&8 (.exe). It asked me to download ff extension even I've got. It couldn't check the ff box in setting, but it works good. The bookmarks are synced.
Interesting😑