Hello, Chacapamac.

Chrome and Firefox should be the only browsers that pop open, and they only do so if you have enabled Sign In to Chrome or Fierfox Sync.  This is necessary to sync with Chrome or Firefox on your other devices.  If you don't need Sign In to Chrome or Fierfox Sync, you can turn these off.

That being said, and I just retested this, browsers should not noticeably pop open nor stay open.  if Chrome or Firefox are not launched and BookMacster launches them to do an export, it quits them after 10 seconds.  Furthermore, it does not bring them forward.  If Chrome or Firefox are running, BookMacster does not bring them forward.  Possibly there are extensions or preferences in Chrome or Firefox which might cause them to come forward when launched.  If you want to tell me more info about which browser(s) "pop open" and stay open, I'll investigate further.

That being said, getting back to your original question, yes you can schedule syncs to occur at specified times instead of within minutes of a bookmarks change.  To do that,

• Activate your Bookmarkshelf document in BookMacster
• Click the tab: Settings > Agents.
• Verify that you have 2-4 checkboxes switched on in the picture.
• Near the top, change the subtab from Simple to Advanced.
• In the Triggers table, there should be one or more Bookmarks Changed triggers.  Delete all of them by clicking the [-] button.
• Add a new trigger by clicking the [+] button.
• When prompted, click Pause.  You need to pause syncing while making adjustments.
• Change the new trigger that has appeared to Scheduled
• Set the Days-of-week and Time as desired
• If you want to sync more than once a day, add additional Scheduled triggers in the same way.  BookMacster may display a warning about Duplicate Triggers if any of your two triggers are the same momentarily.  Just read it and move on to set your day and time parameters.
• To resume syncing with the new triggers, click the Syncing button in the toolbar, then, assuming you didn't change any Content, It's OK.
• Close your document or quit BookMacster.