Firstly, having the tabs at the top is the industry standard, which is why it's done that way in Firefox.

That said, it's still possible to put the tabs at the bottom again in Firefox 65. You could do this previously, but the developers changes the layout of Firefox slightly in the most recent update to Firefox.

Now, you need to have the following code in your userChrome.css file to get it to work properly:

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* only needed once */

/* TABS: on bottom */
#navigator-toolbox toolbar:not(#nav-bar):not(#toolbar-menubar) {-moz-box-ordinal-group:10}
#TabsToolbar {-moz-box-ordinal-group:1000!important}

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

#tabbrowser-tabs {
  width: 100vw !important;
}
#main-window:not([chromehidden*="toolbar"]) #navigator-toolbox {padding-bottom: var(--tab-min-height) !important;}

/* TABS: height */
:root {
 --tab-toolbar-navbar-overlap: 0px !important;
 --tab-min-height: 33px !important; /* adjust to suit your needs */
}
:root #tabbrowser-tabs {
 --tab-min-height: 33px !important; /* needs to be the same as above under :root */
 --tab-min-width: 80px !important;
}

#TabsToolbar {
 height: var(--tab-min-height) !important;
 margin-bottom: 1px !important;
 box-shadow: ThreeDShadow 0 -1px inset, -moz-dialog 0 1px !important;
}

#tabbrowser-tabs,
#tabbrowser-tabs > .tabbrowser-arrowscrollbox,
.tabbrowser-tabs[positionpinnedtabs] > .tabbrowser-tab[pinned] {
  min-height: var(--tab-min-height) !important;
  max-height: var(--tab-min-height) !important;
}

/* drag space */
.titlebar-spacer[type="pre-tabs"],
.titlebar-spacer[type="post-tabs"] {
  width: 40px;
}

/* Override vertical shifts when moving a tab */
#navigator-toolbox[movingtab] > #titlebar > #TabsToolbar {
  padding-bottom: unset !important;
}
#navigator-toolbox[movingtab] #tabbrowser-tabs {
  padding-bottom: unset !important;
  margin-bottom: unset !important;
}
#navigator-toolbox[movingtab] > #nav-bar {
  margin-top: unset !important;
}

/* hide windows-controls */
#TabsToolbar #window-controls {display:none!important;}

/* move caption buttons to right of Tab bar */
#main-window[tabsintitlebar]:not([inFullscreen="true"]) #toolbar-menubar[autohide="true"] ~ #TabsToolbar .titlebar-buttonbox-container {
 position: fixed !important;
 right: 0 !important;
 top: calc(6px + var(--tab-min-height)) !important;
 display: block !important;
 visibility: visible !important;
}
#toolbar-menubar[autohide="true"] ~ #TabsToolbar {padding-right: 100px !important;}

Hope this helps.

See also this thread for updated userChrome.css code for Firefox 65 to move the tabs to the bottom position for more information about the above posted code.

• /questions/1248277 Reposition Tabs at bottom, directly above web page in Firefox 65.0?

I do understand that fine volunteers address this problem regularly. But my question is not that I can research, download, and implement a fix. I can do that, and have done so several times.

My suggestion is that there be a preference -- in about:config or whereever -- to position them in one or the other position.

As an open-source author myself, I do understand the "it's free, be grateful, if it breaks you get to keep both pieces" attitude. But in the browser wars that isn't cutting it. Firefox is losing market share and I believe a small part of it is due to things like this.

I love Firefox and have used it and its predecessors for 24 years. I was tested with Quantum, as so many of the extensions that I had relied on broke. I decided to soldier on. I hope I don't give up and go with Google.

If you want to leave feedback for Firefox developers, you can go to the Firefox Help menu and select Submit Feedback... or use this link. Your feedback gets collected by a team of people who read it and gather data about the most common issues.