After a related frustration with firefox recently my guess is that you can not. At least not in a default way.
Tags and keywords are saved in mozillas proprietary jsonlz4 format which is a one way road. Other bookmark exports do not include this metadata and the only way to import jsonlz4 will overwrite the existing bookmarks.
I really hope some one corrects me, if not consider making a profile (firefox -p or about:profiles)and sync the profile.