I had been using iCLOUD to sync my bookmarks with Safari on iPAD, iPhone w/o any problems with Firefox on my WIN 7/64 PC, using the iCLOUD bookmark.
Some time ago this stopped working and I have no idea what happened. Now my work-around is using FF on iPAD/PHONE, but that is not as convenient as Safari/FF Sync.
iCLOUD otherwise runs fine, also on PC, BUT: in the BOOKMARKS sectionin the iCLOUD “set-up” it only gives Internet Explorer as an option to select. FF cannot be selected, i.e. iCLOUD acts as if FF was not installed at all! But it is, of course, and in its always most recent version.