In FF v70, using this script, I managed to get the "tabs" to move to the bottom of the page, but NOT the "tab bar". Basically making matters worse since you can barely see the area where the reduced width (by 1/3) tab area is;
The result is this;#TabsToolbar {
position: fixed;
bottom: -0.5em;
width: 100%;
}
#tabbrowser-tabs { width: 96%; }
#content-deck { margin-bottom: 2.5em; }
#TabsToolbar:not([inFullscreen="true"]) {position:fixed; bottom:0em; width:100%;}
As opposed to this (in v56 via TabMix Plus) which is what it should be if Mozilla didn't change things and removing the option to place it where it's been for 20 years !
Note the width of the 'correct' tabs in the bar, vs the 'translucent squares' in v70