Are you running Firefox 52 ESR on all those devices? As with the Firefox installation that you posted here with. Or do you have a mix of versions among those devices?

Sync has remained "static" with Firefox 52 ESR, while the Release versions have gotten changes to Sync over the last 8 new versions. Firefox 60 (& ESR 60) received a major change, but that shouldn't have affected Sync as far as deleting bookmarks from the changes I have seen with Fx 60.

I haven't ever used Xmarks but I did try it out when it was called FoxMarks in the days before Chrome came out; didn't care for the Terms of Service with regards to security of user data.

Recommendation: Disconnect from the Sync server on the device you are using before deleting those bookmarks. When you are done re-connect to the Sync server to "send" those changes to the server all at once.

It's been a few years since I did this, but with two separate Profile running in tiled windows side-by-side I would delete a bookmark in one Profile and before I could refocus my eyes on the 2nd Profile the corresponding bookmark there would be deleted.

But once I deleted a few bookmarks fairly quickly that "instantaneous deletion action" stopped working and Sync seemed to change to a "batch mode". IOW, it stopped deleting any additional bookmarks on the Sync server and "forwarding" the changes until I stopped deleting bookmarks altogether and gave it a rest or time to catch up. And I tried that at various hours throughout the day (in continental US "awake" hours) and did see a difference that might be connected to the Sync server loading, so the "batch mode" might be influenced by the load on the server. But that changed a few years ago when the Sync servers were moved to AWS servers.

Thanks for the info. That helps explain what I'm experiencing. And, yes, I do have a mix of Firefox versions at the moment because I'm being cautious about upgrading them to 60.

But after sleeping on my question, I realized that what I'm really wanting to do is the equivalent of the Xmarks capability to do an override-update: either override what's on the server with what's on a device or vice versa, as way of having full control over defining the current state. That's what I really need sometimes but I'm not finding such a capability in Firefox Sync.

Sorry, Mozilla didn't provide for user controls like that with Sync.

About all the user can control is the data types (Bookmarks, Logins, History, etc) that each device sends and receives. The data is merged when it hits the Sync server; no backup is saved for a "previous state" or per device.

As far as how the differences with Mobile device synchronization goes, I have no idea how that is dealt with other than "saved date" for bookmarks, which device they were initially saved on, and the "limit" for Sync storage on each Mobile device. My understanding is that Mobile is fully synced to Desktop, but Mobile doesn't get all the Desktop bookmarks. Similar for synced tabs and browsing history.

There is a new Firefox add-on at Github (an alpha version that was made available within the last few weeks) that supposedly is an extension of Xmarks, but for the life of me I can't recall the name of that extension right now and my search of my browser history hasn't tweaked my recall of it's name.