My excuse for not caring about "incremental" updates is that the WebExtension API gives me limited control over what is stored in which way. The only way to be sure would be to replace Firefox's bookmark management by an independent bookmarks database, similar to what Xmarks does, but that would require a lot of additional complexity for no real advantage (except, maybe, that you can change your bookmarks on multiple computers simultaneously; this is a use case where ymarks is expected to fail).

I just want it to work first, I'll gradually add improvements later. :)