FYI, I discovered that unmatched tags are not included in this feature.  Specifically, after removing some tags within FireFox 45.0.1, and after importing only from FireFox (keeping matched items from FireFox -- BTW, unchecking 'Merge by URL' made no difference), those removed-tags remain in BookMacster's Content. 

I have a workaround around for this shortcoming.  In Agents > Advanced > Commands, I inserted as my first command a 'Delete All Content'.  It may be less efficient, but it produces the desired results.