Sorry, but there are no user controls for when Sync decides when to synchronize of update data beyond "Sync Now".

As far as Bookmarks, Sync adds a new Bookmark almost immediately. But if you would happen to save multiple new bookmarks very quickly Firefox will "sense" that and pause until you stop (controlled within each device / Firefox Profile by the Sync feature in the browser), and then send a batch of new Bookmarks at the same time. Mostly I notice that when moving bookmarks around rather quickly, like when rearranging bookmarks or setting up a new folder and moving multiple bookmarks into that new folder. And similar when deleting multiple bookmarks one at a time; Firefox will pause and then send the changes in a batch once I stop making changes or hesitate "too long". My own observation when I am "testing Sync" between two separate Profiles with their windows tiled side-by-side. Sync works at the Profile level, thus synchronizing Firefox data between each Profile that may use the same Firefox "program" or different installations as long as they are compatible with the Sync server; and of course with a Firefox Profile on a different / separate device.

IMO, part of the "issue" with giving the user control over timing is that the Sync server is shared by many accounts, and depending upon the overall load Sync communications between each device and that users Sync account it needs to be controlled by the server. So when "everyone" is online at the same time Sync will slow down and the "timing" could be considered unpredictable.

As far as browsing history goes, I haven't "experimented" with that at all. My usage of Sync is more to "test" it for user support purposes now-a-days and to try to keep up with changes as they get rolled into Firefox Releases.

And since Quantum Firefox 57, there have been many changes to Firefox basic code to speed up Firefox that has involved "data storage" which involve performance and "temporary" storage of user data in *.shm files; then when Firefox is "Exited" that data gets written to more permanent data files as part of the "exit routine". At that point "updated" info is sent to the Sync server to be shared with other devices / Profiles that are connected to the same Firefox Account / Sync account. And using "Sync Now" will 'push' an update to the Sync server, although I haven't gotten the impression that is helps Sync work better, although surely more "predictable" with user intervention; IMO not needed as long as Firefox is allowed to "exit" properly. IOW, not shutting down or rebooting the Operating System when Firefox is still open or forcing the OS to "close the application".

Server load could explain the variances I have been experiencing just using sync for a few weeks now. At first I blamed the various lag times in my virtual desktop but i have other live paste boards where data appears almost instantly so that negates that theory. I was interested to know if there was something i was overlooking instead I think its user expectation. I have since added the sync button to the menu and have gotten onto the habit now of just hitting it when i expect to see data from the other device - i think that's the way it works sync the machine that is missing the data not the one sending the data if i remember. Works much better for my impatience. I see you posted similar responses previously; I did a search before posting but didn't see anything regarding my specific history issue. Thanks.