Firefox Sync is intended to synchronize data (including bookmarks) across multiple devices. Not much point in using it for one device, as it doesn't provide web access.

Xmarks works a bit differently, and it provides web access to the user's bookmarks.

Well so far I see no evidence that Firefox Sync is syncing, I think I have too many bookmarks.

I did a backup from one Windows Profile to another and it kept giving me script errors.

I installed Xmarks which also took a while but at least I can see they are there and I can see that there is some duplication as there are three or four "bookmarks Toolbars" each of which has my folder structure.

So I guess I can edit the bookmarks online and reduce the number which may make Firefox a bit faster.

There is no total as far as I can see but I seem to remember one of the backups said it had 40,000 which is OTT, I thought I had about 10,000 but if there is this duplication then that makes sense.

The one sure way to see if Sync is working is to setup Sync on one device and let it Sync for quite awhile, and then connect a 2nd device [or another Firefox Profile] to Sync and see what gets sync'd. Keeping in mind that Sync may work in spurts over a long period of time when a lot of data is involved. So Firefox may need to be kept running for some time to get all the data uploaded for the initial Sync and when adding a new device.

10, 000 bookmarks is a whole lot of bookmarks, but I don't think that should keep Sync from working although it make take some time for them all to "sync". I don't recall a maximum number ever being mentioned as the "ceiling" for using Sync.

Having multiple Bookmarks Toolbar folders is normal under certain circumstances, like when importing bookmarks that were exported from another Firefox installation. Sync will sync the BTB bookmarks / folder, but won't mess up the receiving device by changing the BTB folder on that device, but it is easy to select a different Bookmarks Toolbar folder to use as the one that is used on a device. Or possibly by Xmarks being used, although I can't confirm that personally - never used it due to its EULA and lack of privacy as explained in the EULA.

But overall my thoughts are that to have 4 BTB folders, there was probably 3 imports done total. The first may have been via Sync, the 2nd from the export / import, and the 3rd from Xmarks - ??