Firefox favicon / pinned tab size for all tabs by default on open. Add a userChrome.css file within a chrome directory in your profile and restart FF to launch.

userChrome.css

/* 
 * Do not remove the @namespace line, it's required
 */
 @namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* set default namespace to XUL */

/*
 * SETUP:
 * Must go to about:config and change browser.tabs.tabMinWidth to 0. -> this is no longer needed (last checked January 2023)
 */


/* General Rules */
#main-window:not(#f) .tabbrowser-tab {
   -moz-box-sizing: border-box !important;
   height: 33px !important;
   vertical-align: top !important;
}

#tabbrowser-tabs .arrowscrollbox-scrollbox {
  /* overflow: visible !important; */
  display: block;
}

.tabbrowser-tab,
.tab-background {
  height: var(--tab-min-height);
}

.tab-stack .tab-content {
  padding: 0 12px !important;
}

.tabbrowser-tab:not([pinned]) {
 min-width: 40px !important;
 max-width: 40px !important;
}
.tabbrowser-tab:not([pinned]):not([fadein]) {
 max-width: 0.1px !important;
 min-width: 0.1px !important;
}

/* Pinned Tabs Rules */
.tabbrowser-tab[pinned][selected] .tab-content:after{
  /*display: none;*/
  border-color: var(--tab-line-color) transparent transparent transparent;
}

.tabbrowser-tab[pinned][selected] .tab-line {
  display: none !important;
}

.tabbrowser-tab[pinned] .tab-content {
  position: relative;
}

.tabbrowser-tab[pinned] .tab-content:after {
  display: block;
  content: '';
  position: absolute;
  top: 0;
  left: 0;
  width: 0;
  height: 0;
  border-style: solid;
  border-width: 8px 8px 0 0;
  border-color: #ffffff transparent transparent transparent;
  opacity: 0.9;
}

/* NonPinned Tabs Rules */
.tab-icon-image:not([pinned]) {
  margin-left: 0 !important;
  margin-right: 0 !important;
}

.tab-label-container {
  display: none;
}

.tab-close-button {
  display: none;
}

/* THIS IS OTHER STUFF - NOT BEING USED BUT HELPED WITH OVERFLOW ISSUES */

/* #main-window:not(#f) .tabbrowser-tab:not([pinned]) {
   width: auto !important;
}
#main-window:not(#f) .titlebar-button {
   height: 33px !important;
}
#main-window:not(#f) #titlebar-buttonbox,
 .tabbrowser-arrowscrollbox scrollbox > box {
   display: block !important;
}
#main-window:not(#f) #titlebar-buttonbox {
   vertical-align: top !important;
}
.tabbrowser-arrowscrollbox scrollbox {
   overflow: visible !important;
}
.tab-label-container[textoverflow]:not([pinned]) {
   mask-image: unset !important;
} */

userContent.css

@namespace url(http://www.w3.org/1999/xhtml);