Hello, hscott.

I just retested these versions and it's working fine for me.  Steps to reproduce:

•  Run BookMacster 2.1.2.
•  Verify that Syncing is on (green dot in the Syncing button in the toolbar)
•  Quit BookMacster, or at least close that .bkmslf document window.
•  Run Firefox 46.0.1.
•  Add a bookmark.
•  Wait 2 minutes.
•  Run BookMacster.

Result: New bookmark has appeared in .bkmslf Content.

If that's still not working for you, please do this:

•  Add a in Firefox bookmark, noting the exact time in minutes and seconds.
•  Wait 2 minutes.
•  Run BookMacster
•  Click in the menu: BookMacster > Logs….
•  Select the entries (if any, there should be several dozen) beginning after the time you added the bookmark, and drag into an email message to support@sheepsystems.com.
•  Send to us, telling us the time you added the bookmark.

Or you might also check the Sync Logs in the .bkmslf document.  To do that,  click the tab: Reports > Sync Log.  The Sync operations (imports, exports) are displayed in a popup menu near the top of the tab.  You should see an "Import from Firefox" occurring after you added the bookmark, and a few seconds later, probably one or more "Exports" to browser(s).  It's kind of sketchy, but you should see the name of the new bookmark.  You can send those to us too by clicking the "Write to Desktop" button.  Attach the files it writes.

Bookmarks added from Firefox 46.0.1 using BookMacster 2.1.2 and extension 312 are not showing up in my Bookmarkshelf content.  The extension tests OK, and when I do an advanced test adding a Firefox bookmark, the test says BookMacster catches the addition.   However, the new bookmark does not show up in my BookMacster content. Sync is working too.