You're not missing it, Stew.  We've never implemented export to the old Netscape "bookmarks.html" format.  Because Firefox no longer uses it, the only purpose for it is that some older browsers such as Safari use it for importing and exporting.

You can export from BookMacster to Safari (although this will wipe out whatever bookmarks you now have in Safari) and then export from Safari to get a "bookmarks.html".

Unfortunately, web browser designers have generally not embraced interoperable standards, such as the more modern XBEL file format, which BookMacster supports in lieu of the undocumented and ad-hoc Netscape "bookmarks.html".