Syncing a browser is only about the many browser custom settings; extensions, preferences, themes, wallpapers, apps, and addon's, predominantly. Obviously, bookmarks are either used in a cloud setting, and can even be shared cross-browser, and so, 'browser sync' has nothing to do with bookmarks. All major browsers (Chrome, Chromium, FF) sync the browser settings (again, not to be confused with bookmark syncing). I guess Opera will be among the last to do so, or, it will never happen, which will mean it will not be used as much, as in its potential as a front runner browser. Bummer! Thankfully the Linux Mint OS is bringing the Chromium browser back into its repository in mid Dec 2020; at that time I guess I'll swap Opera for it.

Everything is working fine so this is not a problem, as much as it is, confusion on my part. I run Mint 20-C on 3 different units (2 NUC-PCs & a laptop). On my 2 NUCs I have Firefox (default) and Opera as my only web browsers. When I installed Opera on both recently, I wanted to upload all my FF bookmarks, and that was all. Then, I synced the two Opera browsers on the two NUC PCs with an account, so they would share/sync data with one another. Never did I intend to share data between browsers. But today I was looking in the history folder here on Opera browser and saw my Firefox browser history. Not cool! I run separate browsers exactly to run them as completely separate entities. How is this possible?