Are you using code in userChrome.css that is broken and needs an update? This could be a problem with code in userChrome.css that is broken and needs an update. You can check if there is an update available from its creator, there might be a link in the code where you can get the latest version.

• https://www.userchrome.org/what-is-userchrome-css.html

You can set this pref to false in about:config to disable userChrome.css and userContent.css.

• about:config => toolkit.legacyUserProfileCustomizations.stylesheets => false
• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

I have put some commands into user.js; turning off other stuff that I do not like about the browser, so disabling userChrome.css is not an option.

Could you explain a bit more clearly about updating it please? I don't understand anything in the link you posted. I do not see anything about updates or code linking.

Is there anything I can type in about:config or user.js that simply nukes this ugly and unnecessary creation?

Here's what my userChrome.css contains:

/*** BEGIN Firefox 77 (June 2, 2020) Override URL bar enlargement ***/

 /* Compute new position, width, and padding */

 #urlbar[breakout][breakout-extend] {
   top: 5px !important;
   left: 0px !important;
   width: 100% !important;
   padding: 0px !important;
 }
 /* for alternate Density settings */
 [uidensity="compact"] #urlbar[breakout][breakout-extend] {
   top: 3px !important;
 }
 [uidensity="touch"] #urlbar[breakout][breakout-extend] {
   top: 4px !important;
 }

 /* Prevent shift of URL bar contents */

 #urlbar[breakout][breakout-extend] > #urlbar-input-container {
   height: var(--urlbar-height) !important;
   padding: 0 !important;
 }

 /* Do not animate */

 #urlbar[breakout][breakout-extend] > #urlbar-background {
   animation: none !important;;
 }

 /* Remove shadows */

 #urlbar[breakout][breakout-extend] > #urlbar-background {
   box-shadow: none !important;
 }

 /* remove all items from the urlbar drop down */

1. PopupAutoCompleteRichResult {display:none !important;}
2. urlbar-results { display:none!important; }

 /* Hide URL bar drop-down Fx70 */
 #urlbar .urlbarView {
   display: none !important;
 }

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

1. PlacesToolbar #OtherBookmarks .toolbarbutton-text {

display:none !important;

}

1. PlacesToolbar #OtherBookmarks { display:none!important; }

/*** Color the menus (black on white) ***/ menupopup {

 --menu-color: #000 !important;
 --menu-disabled-color: #555 !important;
 --menu-background-color: #fafafa !important;
 --menu-border-color: #333 !important;
 --menuitem-hover-background-color: #ddd !important;

}

1. context-searchselect {display: none !important}

1. navigator-toolbox {

--lwt-tabs-border-color: transparent !important; }

1. tabbrowser-tabs {

--lwt-tab-line-color: transparent !important; }

host, :root {

color-scheme: light light !important; }

/* Directly set colors for mouseover status panel */

1. statuspanel-label {

   color: #000000 !important;
   background-color: #e4e3e3 !important;
   border-color: #e4e3e3 !important;

}

1. alltabs-button { display: none !important; }

/*** END Firefox 77 (June 2, 2020) Override URL bar enlargement ***/