I'm thinking they must hate all browsers then, because Edge and Chrome both do tabs on top. If Firefox had forced the tabs to stay below the address bar by default, the same people would probably be threatening to quit Firefox and go to Chrome because Firefox are living in the past. e.g. - why would it just be tabs that stayed where they are? Everyone has one old setting they liked that's changed in modern browsers. If they tried to keep the "old ways" to keep everyone happy their browser would look like it's 1998.
Personally, I think the idea they're going for is that the top bar is evolving to look like the task bar in windows. So now I mainly just have two black strips at top and bottom, bottom is Windows controls, top is browser controls. The new look Firefox is making Chrome look like retro tech that is seriously clashing with my minimalist all-black Windows 10 theme. The new Firefox with the dark theme turned on actually looks like it's part of the OS.
Personally, what I want that they haven't quite implemented is the Firefox options icon being optionally in the top-left (mirroring Windows Start menu), then a Cortana-like address bar (can shrink or expand it), then shortcut icons / drop-downs, then tabs, then add-ons on the right, mirroring the system tray in Windows. Basically one unified strip of browser controls that mirrors the look and functionality of what is at the bottom of my screen.