I'm not sure who exactly was responsible, but after this block disappeared from my Windows system, I ran into the exploding duplicates problem. I hadn't changed anything anywhere from what had worked in the past, but now certain folders of bookmarks (but not others!) filled up with dozens of exact duplicates on each of my connected systems. I did find that Firefox Sync was set to sync Add-ons, and it had installed iCloud Bookmarks on multiple Windows and Linux systems. I had to disable the iCloud extension everywhere, clean out the duplicates, and disable "Sync Add-ons". Then enable the iCloud extension on only one Firefox machine.
This page describes the problem well:
http://www.democraticunderground.com/109511353
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1. Firefox Sync was set to sync Add-ins as well as bookmarks and settings;
2. The new Firefox iCloud Bookmark sync requires an add-in to be installed in Firefox, which I duly installed on the one PC that I use for this;
3. Unbeknownst to me, Firefox Sync then installed the same add-in on my other desktop PCs (because Firefox Sync was set to sync the add-ins);
4. So at that point I had 3 desktop PCs all syncing bookmarks to iCloud as well as syncing bookmarks with each other;
5. BOOM! Infinite recursive exploding bookmark sync nightmare! (iCloud adds bookmark folder to PC A; PC A syncs this bookmark to PC B; PC B uploads this bookmark to iCloud; iCloud thinks its a new bookmark and sends it to PC A; rinse and repeat.....)
1. Set Firefox Sync not to sync add-ins, on EVERY PC;
2. Remove the iCloud Firefox bookmark sync add-in from all the PCs except the one I use to sync the bookmarks with iCloud (temporarily disable the add-in on this PC);
3. Restore the Firefox bookmarks on one PC from a previous day's automatic backup (Bookmarks-Show all Bookmarks-Import and Backup-Restore). Firefox sync then restored the clean bookmarks to the other PCs.
4. To clean up the bookmarks in iCloud, go to the iCloud control panel and set to sync bookmarks with Internet Explorer (which I never use under normal circumstances). The horrible thousands of bookmarks abomination is then imported into Internet Explorer. But I could then go to the Favorites folder in Windows Explorer and delete them all in just a couple of minutes. So this deleted all the iCloud bookmarks.
5. Get an iPhone and check that the iCloud bookmarks have indeed been deleted.
6. Go back to the iCloud control panel, set to sync bookmarks with Firefox, and re-enable the Firefox add-in to do this.
7. Clean bookmarks are then synced back to iCloud and you are done!
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