ShishiShabbat said

Why are tab positions not optional?

That option was removed in Firefox 29 due to the need to maintain dual coding, one for the default of "tabs on top" and the older UI scheme of "tabs on bottom".

From Fx 29 thru Fx 56 you were probably using the old legacy Classic Theme Restorer which only worked with the Australis versions of Firefox. Quantum doesn't provide for legacy extension such as the Classic Theme Restorer extension.

If you have been using a userChrome.css batch of code with Firefox 57-plus, a slight CSS change was made to the syntax for "tabs". So, you need updated code that works in Fx 65-plus versions.

See this answer: https://support.mozilla.org/en-US/questions/1248277#answer-1192325

Thanks so much for a proper tabs solution. Cheers mate.

W.H.H said

In order to place the tabs directly above the web page windows in Firefox 65 and newer -- that is, below the navigation bar and below any other bars -- copy and paste the following lines into your userChrome.css file and restart Firefox. [Note: Detailed information on using userChrome.css files is available on the https://www.userchrome.org web site maintained by Jefferson Scher (alias: jscher2000).]

:root {
  --tab-min-height_tnot: 32px;
}

#print-preview-toolbar,
#printedit-toolbar,
#titlebar {
  -moz-box-ordinal-group: 0 !important;
}

#navigator-toolbox #toolbar-menubar {
  -moz-box-ordinal-group: 1 !important;
}

#navigator-toolbox #nav-bar {
  -moz-box-ordinal-group: 2 !important;
}

#navigator-toolbox #PersonalToolbar {
  -moz-box-ordinal-group: 3 !important;
}

#navigator-toolbox toolbar {
  -moz-box-ordinal-group: 10 !important;
}

#navigator-toolbox #TabsToolbar {
  -moz-box-ordinal-group: 100 !important;
}

#main-window:not([chromehidden*="toolbar"]) #navigator-toolbox {
  padding-bottom: calc(1px + var(--tab-min-height_tnot)) !important;
}

#TabsToolbar {
  position: absolute !important;
  bottom: 0 !important;
  width: 100vw !important;
}

#tabbrowser-tabs {
  width: 100vw !important;
}

That batch of userChrome.css code does a lot more than just move the Tab Bar. Good way to create additional user support issues for other users who wouldn't want those modifications.

Seems to work (at least for this "update" sic).

Thank you!