Firefox 89 Proton userChrome.css - tabs on bottom attempt - only works with menu bar and bookmarks bar both enabled. See also https://github.com/WesleyBranton/Remove-Firefox-Megabar/blob/master/remove_megabar.css

userChrome.css

@import "megabar_kill.css";

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

/* Mostly works as of Firefox 89 (proton). */


/*
#urlbar-container, #search-container{
  padding-block: 2px !important;
}

#urlbar-container, #urlbar-container[breakout], #searchbar-container, #searchbar-container[breakout]{
  min-height: 24px !important;
  --urlbar-container-height: 24.00px !important;
}

* {
  font-family: "Tahoma NoAA",Tahoma,"Liberation Sans",Arial,Helvetica,sans-serif !important;
  --urlbar-container-height: 24.00px !important;
  */
* {
  font-family: "Tahoma NoAA",Tahoma,"Liberation Sans",Arial,Helvetica,sans-serif !important;
}

/* :root {
  font: 8pt "Tahoma NoAA",Tahoma,"Liberation Sans",Arial,Helvetica,sans-serif !important;
} */

window, page, dialog, wizard, prefwindow {
  font: 8pt "Tahoma NoAA",Tahoma,"Liberation Sans",Arial,Helvetica,sans-serif !important;
}

/* bar on top of selected/highlighted tab */
/* #tabbrowser-tabs */


.tab-close-button,.tab-close-button > .toolbarbutton-icon {
   display: -moz-box !important;
}   

/* no stupid click-drag in menus moving the entire window */
#navigator-toolbox {
  -moz-window-dragging: no-drag !important;
}

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

/* info bar, currently just set to hover because I can't figure
   out how to make it not mess up the tabs otherwise */
.notificationbox-stack:not([inFullscreen="true"]) {
  /* for alerts like pop-up blocked */
  -moz-box-ordinal-group: 1500 !important;
  display: block !important;
  position: absolute !important;
  top: calc(var(--tabbar-top) + 25px)  !important; 
}

.tab-background {
  display: none !important;
  border-bottom: 1px solid #797C88 !important;
}

.tabbrowser-tab {
    /* no rounded corners */
  --tab-border-radius: 0px !important;
  border-top: 1px solid #DDDEE6 !important;
  border-bottom: 1px solid #797C88 !important;
}

/* active tab fighting */

.tabbrowser-tab:not([visuallyselected=true]){
  /* appearance: none !important; */
  /* border-right: 0px solid #797C88 !important;*/
  /* 89+ needs a left border */
  border-left: 0.5px solid #797C88 !important;
  /* background-color: #CCD0DD !important;*/
}

.tab-background[selected="true"] {
  background-image: none !important;
}

.tab-background {
  box-shadow: none !important;
}

/* current tab background color */
.tabbrowser-tab[visuallyselected=true]{
  font-weight: bold !important;
  background-color: #BBC0D1 !important;
}

.tabbrowser-tab[visuallyselected=true] .tab-background {
  border: 0px none transparent !important;
  background-color: #BBC0D1 !important;
}

.tab-icon-image[fadein], .tab-close-button[fadein], .tabbrowser-tab[fadein]::after, .tab-background[fadein]{
  transition: none !important;
}

.tabbrowser-tab[visuallyselected=true] {
  /* background-color: #EDF1FF !important; */
  border-top: 1px solid #DDDEE6 !important;
  /* border-right: 0.5px solid #797C88 !important; */
  /* 89+ needs a left border */
  border-left: 0.5px solid #797C88 !important;
  border-bottom: 0.5px solid #797C88 !important;
 /* border: .5px solid #565964 !important;
  border-top: transparent !important;*/
}

.tab-line {
  background-color: #DDDEE6 !important;
  height: 0px !important;
  /* --tab-line-color: none !important;*/
}

/* Tabs on bottom */
*|*:root {
 --menubar-height: 36px;
 --bookmarkbar-height: 30px;
  /* ff 89+ */
 --tabbar-top: calc(var(--menubar-height) + var(--bookmarkbar-height) + var(--tab-min-height) - 11px); /*89+*/
  --tab-min-height: 25px !important;
  --tab-max-height: 25px !important;
}

#TabsToolbar:not([inFullscreen="true"]) {
  --tab-min-height: 25px !important;
  --tab-max-height: 25px !important;
  -moz-box-ordinal-group: 1000 !important;
  display: block !important;
  position: absolute !important;
/* bottom: 0 breaks on ff89 */
/*  bottom: 0 !important; */
  top: var(--tabbar-top) !important;
/*  width: 100vw !important; */
}
.tab-background {
  background-color: rgba(0,0,0,0) !important;
  --proton-tab-block-margin: 0 !important;
}



/* end tabs on bottom */
.tabbrowser-tab[visuallyselected=true]::after {
  /* Shadow was too big on the left side of active tab otherwise */
  border-left: none !important;
}

.tabbrowser-tab {
  padding-inline: 0px !important;
}

.tabbrowser-tab::after, .tabbrowser-tab::before {
  /* tab separator lines to full height */
  margin-block-start: 100% !important;
  margin-block-end: 100% !important;
}

*|*:root[tabsintitlebar]:not([inFullscreen="true"]):not([sizemode="maximized"])
 #toolbar-menubar[autohide="true"] ~ #TabsToolbar{
  bottom: var(--tab-min-height) !important;
  padding-top: calc(var(--tab-min-height) - 22px) !important; /*adjust*/
}

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

/* navigator-toolbox - padding */
*|*:root:not([chromehidden*="toolbar"]):not([inFullscreen="true"]) #navigator-toolbox {
  padding-bottom: calc(var(--tab-min-height) + 1px) !important; /*adjust*/
}

/* TABS: height */
*|*:root {
  --tab-toolbar-navbar-overlap: 0px !important;
  --tab-min-height: 25px !important; /* adjust */
  --tab-min-width: 80px !important; /* adjust */
}

#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;
}

#TabsToolbar {
  height: var(--tab-min-height) !important;
/*  margin-bottom: 1px !important;*/
  box-shadow: ThreeDShadow 0 -1px inset, -moz-dialog 0 1px, ThreeDLightShadow 0 1px !important; /* omit */
  background-color: var(--toolbar-bgcolor) !important;
  color: var(--toolbar-color) !important;
}

.tab-background[multiselected="true"], .tab-background[selected="true"] {
  /*  border-top-color: var(--tabs-border-color);*/
  border-top-color: none !important;
}

  /* tab-top color */
/*#tabbrowser-tabs {
  --tab-line-color: #666 !important;
}*/

/* indicators */
*|*:root[privatebrowsingmode=temporary] .private-browsing-indicator {
  position: absolute !important;
  right: 0px !important;
  bottom: 0px !important;
  width: 14px !important;
  pointer-events: none !important; /* click-through */
}

/* makes everything in toolbars a little more compact */
*|*:root {
  --toolbarbutton-outer-padding: 0px !important; /* was 6px. 0px is super condensed. */
  --urlbar-icon-padding: 2px !important;
}

/* Drag Space */
.titlebar-spacer[type="pre-tabs"],
.titlebar-spacer[type="post-tabs"] {
  width: 20px !important;
}

/* 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;
}

/* indicators - HIDE */
.private-browsing-indicator {display: none !important;}
.accessibility-indicator    {display: none !important;}

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

/* caption buttons - HIDE */
*|*:root[tabsintitlebar]:not([inFullscreen="true"])
 #toolbar-menubar[autohide="true"] ~ #TabsToolbar
 .titlebar-buttonbox-container {display: none !important;}

/* find bar on top */
.browserContainer > findbar {
  -moz-box-ordinal-group: 0;
  transition-duration: 0s, 0s, 0s !important;
  transition-timing-function: unset !important;
  transition-property: unset !important;
  border-top-width: 0px !important;
  /*  border-bottom: 1px solid var(--toolbox-border-bottom-color) !important;*/
  border-bottom: 1px solid ThreeDShadow !important;
}

findbar {
  padding-block: 0px !important;
}

.findbar-find-previous, .findbar-find-next{
  padding: 3px !important;
}

.findbar-closebutton {
  /* on left side */
  -moz-box-ordinal-group: 0;
  /* less padding */
  padding: 0 2px !important; /* default '0 8px' */
  margin: 2px !important; /* defaults to 8px in FF 88 */
}

#nav-bar-customization-target > :is(toolbarbutton, toolbaritem):first-child, #nav-bar-customization-target > toolbarpaletteitem:first-child > :is(toolbarbutton, toolbaritem){
  padding-inline-start: 2px !important;
}

.findbar-highlight, .findbar-case-sensitive, .findbar-match-diacritics, .findbar-entire-word, .findbar-label{
  margin: 0 3px !important; /* default '0 8px' */
}

.findbar-container {
  margin-inline-start: 0px !important;
  padding-inline-start: 0px !important;
  /* to reduce vertical height we can also change 'padding-block' */
  padding-block: 1px !important; /* defaults to 4px */
  height: 24px !important;
}

/* first tab left side border missing when new tab button moved over there */

.tabbrowser-tab[last-visible-tab]{
  border-right: 0.5px solid #797C88 !important;
}

/* .tabbrowser-tab[first-visible-tab]:not([selected])::before{(*/
.tabbrowser-tab[first-visible-tab][visuallyselected]::before
{
  content: "";
  display: -moz-box;
  /* border-left: 0.5px solid #797C88 !important;*/
  /*border-left: 1px solid !important;*/
/*  border-color: var(--tabs-border-color) !important;*/
  margin: 0;
  opacity: 1;
}
.tabbrowser-tab[first-visible-tab]:not([selected])::before
{
  content: "";
  display: -moz-box;
  /*border-left: 1px solid !important;*/
/*  border-color: var(--tabs-border-color) !important;*/
  margin: 0;
  opacity: 1;
}


/*.tabbrowser-tab[first-visible-tab][selected]::before{
  content: "";
  display: -moz-box;
  border-left: 1px solid !important;
  margin: 0;
  opacity: 0.3;
}*/

/*.tabbrowser-tab[first-visible-tab][visuallyselected]::before{
  border-color: var(--tabs-border-color) !important;
  border-left: 1px solid !important;
  opacity: 1 !important;
  display: -moz-box !important;
}*/


/*.tabbrowser-tab:not([selected])::after, .tabbrowser-tab:not([selected])::before {
    border-left: 1px solid var(--lwt-background-tab-separator-color, currentColor) !important;
    margin-block: 5px 4px !important;
}*/

/* .tabbrowser-tab::before, .tabbrowser-tab::after { */
  /*    margin-inline-start: -1px !important;*/
/*   margin-inline-start: 1px !important; */
/* } */

.tabbrowser-tab[visuallyselected]::before, #tabbrowser-tabs:not([movingtab]) > #tabbrowser-arrowscrollbox > .tabbrowser-tab[beforeselected-visible]::after, #tabbrowser-tabs[movingtab] > #tabbrowser-arrowscrollbox > .tabbrowser-tab[visuallyselected]::before, .tabbrowser-tab[visuallyselected]::after
{
  border-color: var(--tabs-border-color) !important;  
}

/* new tab button stays on left side of all tabs */
#tabs-newtab-button, #TabsToolbar #new-tab-button {
  -moz-box-ordinal-group: 0;
}

/*** Revert tab throbber - for Firefox 80 as of 8/28/2020 ***/
.tab-throbber[busy] {
  background-image: url("chrome://global/skin/icons/loading.png") !important;
  animation: unset !important;
}
.tab-throbber[busy]:not([progress]) {
  /* Grays the blue during "Connecting" state */
  filter: grayscale(100%);
}
@media (min-resolution: 2dppx) {
  .tab-throbber[busy] {
    background-image: url("chrome://global/skin/icons/[email protected]") !important;
  }
}
/* Suppress bouncing dots when ui.prefersReducedMotion = 0 */
.tab-throbber[busy]::before {
  background-image: none !important;
  animation: unset !important;
}
@media (min-resolution: 2dppx) {
  .tab-throbber[busy]::before {
    background-image: none !important;
  }
}

/* remove hamburger icon cancer */
#PanelUI-menu-button {
  display: none !important;
}

/* remove URL bar "reader view" and "page actions" buttons, so that a single
   tab key press will get me to the search bar from the URL bar
   ( ctrl+L, <tab> )*/
#page-action-buttons {
  display: none !important;
}

/* smaller gap */
#urlbar-search-splitter {
  min-width: 8px !important; /* default 12px */
}

/* disable "list all tabs" dropdown button that appears when lots of tabs are opened */
#alltabs-button {
  display: none !important;
}

#urlbar-background, #searchbar{
  box-shadow: none !important;
}
#urlbar[focused="true"]:not([suppress-focus-border]) > #urlbar-background {
  /* not an accurate color, I just picked something close enough out of laziness */
  border: 1px solid #505050 !important;
}