Give the option for both. If that confuses or scares users too much, have one as the default and the other as an add-on that users who really want it will have

Give the option for both. If that confuses or scares users too much, have one as the default and the other as an add-on that users who really want it will have to download. As of right now, ff users have to settle for the tabs-on-bottom style unless they want to settle for a combination of 5 add-ons, which - i'll assume - significantly increases memory usage.