How do I know? By the fact that there are millions of FF users who want the old control back... the old control that allows the adjusting of where to place the Tabs bar, above or below the Bookmarks bar.
We seem to be locked into Tabs above the Address and the Bookmarks bars. Oh sure. We can go through the trouble of making a Chrome folder, and placing a
userChrome.css file with the proper scripting into it. But why won't the FF people simply add a simple control in preferences or tools like they had years ago?
I surmise that it is because there is one stubborn FF joker running the show, and he/she simply wants to show us who is boss.
What do you think?