For most browsers, we can get the new bookmark information from the browser using AppleScript.  But Firefox is not AppleScriptable, so instead we must use one of our browser extensions.

Either the BookMacster Sync or BookMacster Button extension, installable from the menu BookMacster > Manage Browser Extensions… window, will suffice.  If you do not need BookMacster to sync Firefox bookmarks, you should install the BookMacster Button extension because it is more lightweight than BookMacster Sync.

If you already do have one of these extensions installed, maybe it is not working.  Uninstall and then re-Install.

I just tried adding a bookmark from Firefox and it works for me.

I get the impression that you are adding a bookmark to Safari, waiting for BookMacster to sync it in, then launching BookMacster to add your tag.

BookMacster offers six facilities for adding a new bookmark directly to BookMacster.  These are listed in the Help Book.  I recommend one of the first three (global keyboard shortcut, dock menu or menu extra).  (The fourth method (BookMacster Button) is not available in Safari because of the limited capabilities Apple has provided for Safari extensions.)

Then, to add tags with minimum clicking, either set the heading in the third column of your Content view to show "Tags" and use the popover, or if you are like me and don't like popovers, in Preferences > Adding, set the bullets to open the Inspector upon landing a new bookmark.