IMHO the perfect layout is (vertically speaking):
Title bar (let the OS draw this e.g. obeying users ability to choose Windows accent colour)
Menu bar (except on Mac where menu bars live at the top of the screen)
Navigation bar (with dedicated search box by default, and no hamburger menu - traditional menus are a more efficient use of space)
Bookmarks bar (user selectable, maybe have option to show only when user has added bookmarks to it)
Tab bar (looking like tabs, not buttons, clear dividers between inactive tabs so they don't look like "one thing")
Content area (let OS draw scroll bar elements or at least use OS supplied UI elements)
I switch between tabs A LOT, much more than using any other UI element, and my mouse pointer is often within the content area so having tabs at the "bottom of the toolbar stack" (as they were when they first appeared) would be a huge benefit for usability.
Every update Mozilla likes to break the custom css I had to put tabs back on the bottom. I've gotten so annoyed I stopped updating my firefox.
i too want the tabs on the bottom of the screen, just above the taskbar.
there used to be an addon"Tabmix Plus" that could do that as well as other changes.
i am running ff 94.0 and have disabled updates and update notifications because i am afraid they would
break my userchrome script that put the tabs on the bottom.
tabs on the bottom is intuitively a better location than the top of the screen.
wish there was an addon that could do that for ff 99+