Hello, Ste.

Thank you for contacting us.  Unfortunately, we don't support directly importing those old Netscape bookmarks .html files.  When developing BookMacster in 2009, we did the forward-looking thing and ignored those in favor of the newer .xbel files.  But .xbel files never took off

The workaround is to use Safari, which is still in the twentieth century.  Run Safari and click in the menu: File > Import from > Bookmarks HTML File….  It will put everything in a folder named Import <date>.  Safari can handle thousands of bookmarks, and it is easy to delete this folder when you are done.  It may take tens of seconds; be patient.  Then you can activate one of our apps and File > Import from only… > Safari.  Since you are using BookMacster, you may do this in a File > New Bookmarkshelf/Collection to avoid messing up your regular bookmarks file.  Once they are in our apps, you can find and eliminate duplicates, and ultimately copy or drag to other documents, or export to browsers.