Indeed, there was an interesting little bug, which would cause any change which OS X had not yet autosaved (which it does every 5 minutes, or when you switch apps) to be lost upon backgrounding.  The bug has now been fixed in version 1.22.23.

Version 1.22.23 will be available "soon", which means we don't really know yet.  Version 1.22.23 has been a couple months in the making, and has many bug fixes and performance improvements.  But therefore there is a lot to test 

Thx Jerry, you are not "backgrounding" bookmacster in between and you are using the bookmacster system menu instead of FF's / Bookmarks / Add & Inspect.

The issue is only reproducible with bookmacster "backgrounded" right after the new folder has been added. If the window is kept open and visible the new folder is always available.

Well, it's just that BookMacster can be used in so many ways, I incorrectly assumed the other way in this case.  I understand what you mean now and shall check it out.

Hello, sheepcore.

New soft folders, or other items, that you add to a .bkmslf document in BookMacster will not show in Firefox until after you have executed a File > Export to Firefox.

If you have been using BookMacster as a Central Store for your bookmarks, you may not be familiar with BookMacster's Import and Export functions, so let me explain a few things.  If Firefox is one of the document's Clients, a File > Export to all (…, Firefox, …) will, unless you have adjusted Advanced Client Settings, replace whatever you have in Firefox, deleting any bookmarks which do not exist in the .bkmslf document.  A File > Export to only > Firefox will give you the choice of this Normal export or an Overlay.  An Overlay will not delete any existing items in Firefox.

You can read more about Import and Export in sec. 4.3 of the Help Book.