You need to create a userChrome.css file to do this

• https://www.userchrome.org/what-is-userchrome-css.html

After you've done that, copy / paste the code into your userChrome.css file, save it, and restart Firefox.

   /* Tab bar below Navigation & Bookmarks Toolbars 
  For best results, show Title Bar or Menu Bar */
  #nav-bar { /* main toolbar */
  -moz-box-ordinal-group: 1 !important;
  border-top-width: 0 !important; 
   }
    #PersonalToolbar { /* bookmarks toolbar */
   -moz-box-ordinal-group: 2 !important;
   }
   #TabsToolbar { /* tab bar */
 -moz-box-ordinal-group: 3 !important;
   }

This does not work. I need the tabs on the very bottom of the screen not to just move below address bar

/* Required for the tab bar to be visible down the window */ #content-deck { position: relative !important; } /* Makes sure the halo effect on the tabs bar appears below content */ #navigator-toolbox { overflow: initial !important; } /* Move tabs bar down */ #main-window[tabsintitlebar] #TabsToolbar { transform: translateY( calc(100vh - var(--tab-min-height)) ) !important; } /* Move tabs bar down when the profile has `browser.tabs.drawInTitlebar` set to `false` */ #main-window:not([tabsintitlebar]) #TabsToolbar { transform: translateY(100vh) !important; } /* Remove margin at the top since the tab bar is going down */ #main-window[tabsintitlebar] #navigator-toolbox:not([fullscreenShouldAnimate]) { margin-top: calc(var(--space-above-tabbar) - var(--tab-min-height)) !important; } /* Remove margin at the top when the profile has `browser.tabs.drawInTitlebar` set to `false` */ #main-window:not([tabsintitlebar]) #navigator-toolbox:not([fullscreenShouldAnimate]) { margin-top: calc(0px - var(--tab-min-height)) !important; } /* Maximized window requires different margin to make room for minimize/restore/close buttons. */ #main-window[tabsintitlebar][sizemode="maximized"] #navigator-toolbox:not([fullscreenShouldAnimate]) { margin-top: calc(var(--space-above-tabbar) - var(--tab-min-height) + 12px) !important; } /* This adds padding to the bottom of the window to make room for the tabs bar. */ #main-window:not([sizemode="fullscreen"]) #browser-panel { padding-bottom: var(--tab-min-height) !important; }

Lilcap18 said

Why do you keep removing the option for tabs on the very bottom of the browser window? Firefox 57 is useless omitting that feature.

I don't remember that ever being a built-in feature. I actually only heard someone mention they used it in the past couple of months. It would be hard after all these years to retrain myself to look at the bottom of the screen, so I guess I know how you feel about having to look at the top of the screen.

Hope the style rule works.

Thank You Moses that worked. The tab bar is on the very bottom now. Any idea how to change the tab background back to being the theme and or rounding the tabs?

The bar is on the bottom but its a solid color gray. Jscher2000 it was an option and then it available using add ons "tabs mix plus" which allows you to edit the tabs in all sorts of ways or with "tabs on bottom". The reason I need them on the bottom is because I am using a very large display (49 inch 4k television) so its very hard to see above my line of site. Seems like a very simple set of options that should be available in settings. Quantum seems to not allow any add ons or extensions.

Aha, I never used Tab Mix Plus. Quantum allows a wide range of extensions, but not to modify the user interface in major ways.