Firefox 91 userChrome.css

userChrome.css

/* Remove the gap to the left and right of the title tab bar */
#TabsToolbar hbox.titlebar-spacer[type="pre-tabs"],
#TabsToolbar hbox.titlebar-spacer[type="post-tabs"] {
  display: none !important;
}

/* Remove the buttons on the right of the location bar */
#page-action-buttons {
  display: none !important;
}

/* Proton Tabs Tweaks */
#tabbrowser-tabs {
  --user-tab-rounding: 6px;
}
.tab-background {
  border-radius: var(--user-tab-rounding) var(--user-tab-rounding) 0px 0px !important;
  margin-block: 1px 0 !important;
}
/* Remove extra padding between tabs */
.tabbrowser-tab {
  padding-left: 0 !important;
}
#TabsToolbar .toolbarbutton-1 {
  /* Remove rounded corners from new tab / list all tabs button */
  --tab-border-radius: 0;
  /* Make the new tab button touch the toolbar */
  --tabs-navbar-shadow-size: 0;
}
/* Remove extra padding in tab scroll buttons */
#scrollbutton-up,
#scrollbutton-down {
  padding: 0 !important;
}

/* Tighten up drop-down/context/popup menu spacing */
menupopup > menuitem, menupopup > menu {
  padding-block: 4px !important;
}
:root {
  --arrowpanel-menuitem-padding: 4px 8px !important;
}