My personal Firefox userChrome.css to make Proton feel more like Quantum - Restore connected/square tabs, modify tab background color and text color, tighten up hamburger and context menus, change color of bookmark folders, disable search bar dropdown

userChrome.css

/* Firefox userChrome.css */

/*** PROTON TABS TWEAKS ***/
/* SOURCE: modified version of https://www.userchrome.org/firefox-89-styling-proton-ui.html#tabstyler */
/* Make tab shape square */
#tabbrowser-tabs {
  --user-tab-rounding: 0px;
}

.tab-background {
  border-radius: var(--user-tab-rounding) var(--user-tab-rounding) 0px 0px !important;
  margin-block: 1px 0 !important;
}

/* Borders on tab scroll right and left buttons */
#scrollbutton-up, #scrollbutton-down { /* 6/10/2021 */
  border-top-width: 1px !important;
  border-bottom-width: 0 !important;
}

/* Container color bar visibility */
.tabbrowser-tab[usercontextid] > .tab-stack > .tab-background > .tab-context-line {
  margin: 0px max(calc(var(--user-tab-rounding) - 3px), 0px) !important;
}

/* Inactive tabs: Separator line style */
/* For light backgrounds */
.tabbrowser-tab:not([selected=true]):not([multiselected=true]):not([beforeselected-visible="true"]) .tab-background {
  border-right: 1px solid var(--lwt-background-tab-separator-color, rgba(0, 0, 0, .20)) !important;
}

/* For dark backgrounds */
[brighttext="true"] .tabbrowser-tab:not([selected=true]):not([multiselected=true]):not([beforeselected-visible="true"]) .tab-background {
  border-right: 1px solid var(--lwt-background-tab-separator-color, var(--lwt-selected-tab-background-color, rgba(255, 255, 255, .20))) !important;
}

.tabbrowser-tab:not([selected=true]):not([multiselected=true]) .tab-background {
  border-radius: 0 !important;
}

/* Remove padding between tabs */
.tabbrowser-tab {
  padding-left: 0 !important;
  padding-right: 0 !important;
}

/* Set tab fill color and text color */
#TabsToolbar {
  background-color: #202340;
  color: #F9F9FA;
}
/*** END PROTON TABS TWEAKS ***/


/*** TIGHTEN UP DROP-DOWN/CONTEXT/POPUP MENU SPACING ***/
/* SOURCE: https://www.userchrome.org/firefox-89-styling-proton-ui.html#menuspacing */
menupopup > menuitem, menupopup > menu {
  padding-block: 4px !important;
}

/* Tighten up hamburger menu spacing and square the edges */
:root {
  --arrowpanel-menuitem-padding: 2px !important;
  --arrowpanel-border-radius: 0px !important;
  --arrowpanel-menuitem-border-radius: 0px !important;
}
/*** END TIGHTEN UP DROP-DOWN/CONTEXT/POPUP MENU SPACING ***/


/* Fill new style bookmark folders with specified color */
/* This also works when using "show all bookmarks" */
/* This does not require the image file "folder-item.png" inside the chrome folder */
.bookmark-item[container], treechildren::-moz-tree-image(container) {
  fill: #e67a17 !important;
}

/* Hide dropdown that appears when you type in search bar */
.autocomplete-history-popup,
panel[type=autocomplete-richlistbox],
panel[type=autocomplete] {
  display: none !important;
}