Try to ask advice at the Firefox CSS subreddit forum.

• https://www.reddit.com/r/FirefoxCSS/

You would have to use an autoconfig.cfg file to disable a built-in keyboard shortcut.

• https://support.mozilla.org/en-US/search?product=firefox&w=2&q=%22chrome-document-global-created%22

You can use the Browser Toolbox to find the ID of the shortcut you want to disable or override.

• https://developer.mozilla.org/en-US/docs/Tools/Browser_Toolbox

I found a way by adding two files under the Firefox directory

• • Firefox64\defaults\pref\config-prefs.js**

   pref("general.config.filename", "config.js");    
   pref("general.config.obscure_value", 0);  
   pref("general.config.sandbox_enabled", false); 

• • Firefox64\config.js**

   let { classes: Cc, interfaces: Ci, manager: Cm  } = Components;
   const {Services} = Components.utils.import('resource://gre/modules/Services.jsm');
   function ConfigJS() { Services.obs.addObserver(this, 'chrome-document-global-created', false); }
   ConfigJS.prototype = {
    observe: function (aSubject) { aSubject.addEventListener('DOMContentLoaded', this, {once: true}); },
    handleEvent: function (aEvent) {
      let document = aEvent.originalTarget; let window = document.defaultView; let location = window.location;
      if (/^(chrome:(?!\/\/(global\/content\/commonDialog|browser\/content\/webext-panels)\.x?html)|about:(?!blank))/i.test(location.href)) {
        if (window._gBrowser) {
          let keys = ["key_find", "key_findAgain", "key_findPrevious", "key_gotoHistory", "addBookmarkAsKb", "bookmarkAllTabsKb", "showAllHistoryKb", "manBookmarkKb", "viewBookmarksToolbarKb", "key_savePage", "key_search", "key_search2", "focusURLBar", "focusURLBar2", "key_openDownloads", "openFileKb", "key_reload_skip_cache", "key_viewSource", "key_viewInfo", "key_privatebrowsing", "key_quitApplication", "context-bookmarklink"];
          for (var i=0; i < keys.length; i++) {
             let keyCommand = window.document.getElementById(keys[i]);
             if (keyCommand != undefined) { 
                keyCommand.removeAttribute("command"); 
                keyCommand.removeAttribute("key"); 
                keyCommand.removeAttribute("modifiers"); 
                keyCommand.removeAttribute("oncommand"); 
                keyCommand.removeAttribute("data-l10n-id"); 
             }
          }
        }
      }
    }
   };
   if (!Services.appinfo.inSafeMode) { new ConfigJS(); }

You can get a list of ids for the keys from the source by putting the following URL in your browser

   view-source:chrome://browser/content/browser.xhtml

Note that in Firefox 117 and newer you need to remove this line:

const {Services} = Components.utils.import('resource://gre/modules/Services.jsm');

Add this line instead: const Services = globalThis.Services || ChromeUtils.import("resource://gre/modules/Services.jsm").Services;