Я просто нагуглил эту репу -  https://github.com/Aris-t2/CustomCSSforFx/releases

Потом удалил из нее строчку за строчкой всё лишнее, и в итоге получился такой userChrome.css (первые три переменные я сам добавил):

:root {
  --classic_square_tabs_active_tab_background: #eaeaea;
  --classic_square_tabs_active_tab_foreground: black;
  --classic_square_tabs_inactive_tab_background: #bebebe;
  --classic_squared_tabs_tab_height: 26px;
  --classic_squared_tabs_active_tab: linear-gradient(to top,#f9f9fa,#f9f9fa,#f9f9fa);
  --classic_squared_tabs_other_tabs: linear-gradient(to top,#aeaba5,#c1beb7,#c9c6be);
  --classic_squared_tabs-border_size: 1px;
  --classic_squared_tabs-border1: #5f7181;
  --classic_squared_tabs-border2: rgba(0,0,0,.2);
  --classic_squared_tabs-border3: rgba(0,0,0,.5);
  --classic_squared_tabs_new_tab_icon_color: black;
  --classic_squared_tabs_tab_text_color: black;
  --classic_squared_tabs_tab_text_shadow: transparent;

  --tab-min-height: var(--classic_squared_tabs_tab_height) !important;
  --mltabs-newtab-height: calc( var(--classic_squared_tabs_tab_height) + 1px ) !important;
  --tab-min-height_tnot: calc( var(--classic_squared_tabs_tab_height) - 1px ) !important; /* for tabs_below_navigation_toolbar_alt.css */
}

/* default tabs color */
:root:not([lwtheme]) #TabsToolbar :is(.tabs-newtab-button,#tabs-newtab-button),
:root:not([lwtheme]) #TabsToolbar .tabbrowser-tab .tab-content {
  background-color: var(--classic_square_tabs_inactive_tab_background) !important;
}

/* selected tabs color */
:root:not([lwtheme]) #TabsToolbar .tabbrowser-tab .tab-content:is([selected],[multiselected]) {
  background-color: var(--classic_square_tabs_active_tab_background) !important;
}

.tabbrowser-tab[selected="true"] .tab-label {
  color: var(--classic_square_tabs_active_tab_foreground) !important;
}

/* tab border color */
:root:not([lwtheme]) #TabsToolbar :is(.tabs-newtab-button,#tabs-newtab-button),
:root:not([lwtheme]) #TabsToolbar .tabbrowser-tab .tab-content {
  border-top: var(--classic_squared_tabs-border_size) solid var(--classic_squared_tabs-border1) !important;
  border-left: var(--classic_squared_tabs-border_size) solid var(--classic_squared_tabs-border1) !important;
  border-right: var(--classic_squared_tabs-border_size) solid var(--classic_squared_tabs-border1) !important;
}

.tab-background {
  margin-block: unset !important;
}

/* width of new tab tab */
#TabsToolbar :is(.tabs-newtab-button,#tabs-newtab-button){
  min-width: 0px !important;
  width: 28px !important;
  margin-bottom: -1px !important;
}

.tab-close-button {
  margin-inline-end: -6px !important;
  margin-inline-start: 2px !important;
  width: 14px !important;
  height: 14px !important;
  padding: 2px !important;
  border-radius: unset !important;
}

/* size of new tab tabs '+' icon */
#TabsToolbar :is(.tabs-newtab-button,#tabs-newtab-button) .toolbarbutton-icon{
  min-width: 0px !important;
  min-height: 0px !important;
  width: 14px !important;
  height: 14px !important;
  margin: 0px !important;
  margin-bottom: 0px !important;
  padding: 0px !important;
  background: unset !important;
  box-shadow: unset !important;
}
.tabbrowser-tab {
  padding-inline: initial !important;
}

/* restore border between navigation toolbar and tabs toolbar */
:root:not([lwtheme]) #nav-bar {
  box-shadow: 0 calc(-1 * var(--tabs-navbar-shadow-size)) 0 var(--classic_squared_tabs-border2) !important;
}

/* Fx 97+ height fix */
#tabbrowser-tabs:not([secondarytext-unsupported]) .tab-label-container {
  height: unset !important;
}

* {
  border-radius: 0 !important;
  box-shadow: none !important;
}

И добавил в конец немного моих украшательств:

#urlbar, #urlbar-background, #searchbar:focus-within {
  outline:none !important;
  background-color: cornsilk !important;
  border-color: var(--toolbar-field-focus-border-color) !important;
}

#nav-bar, #navigator-toolbox {
  box-shadow: none !important;
  border-top: none !important;
  background-color: var(--classic_square_tabs_active_tab_background) !important;
}