Sorry Mozilla Fireox does not do support for Apple. They have their own support site you can talk to since it is that service that is your problem.

You can do this and just back them up, put on usb, email to your self.... https://support.mozilla.org/en-US/kb/export-firefox-bookmarks-to-backup-or-transfer and https://support.mozilla.org/en-US/kb/import-bookmarks-html-file

Please let us know if this solved your issue or if need further assistance.

Is there another way to sync my PC Firefox 57.0 bookmarks in iCloud?

Ok but just to let you know it is more secure to back up your own and email it to yourself. as per "The extension stopped working. Now my bookmarks are out of sync between Firefox on Windows and my Apple devices. When it comes time for bookmark cleanup, I have to do it twice. This is a real bummer!"

No idea, do not own a Iphone but maybe this is the info looking for.

Would be sync between desktop and Firefox for Iphone only I am thinking.

https://support.mozilla.org/en-US/kb/sync-firefox-bookmarks-and-browsing-history-iOS

Would need to read info form the Extension or the support page : https://addons.mozilla.org/en-US/firefox/addon/icloud-bookmarks/

No idea as page is 4 yrs old https://www.howtogeek.com/175294/how-to-sync-any-browsers-bookmarks-with-your-ipad-or-iphone/

Please let us know if this solved your issue or if need further assistance.