You can't disable a keyboard shortcut via CSS (you can only hide content), that is only possible via JavaScript and can possibly be achieved via an autoconfig.cfg file.

You can't disable a keyboard shortcut via CSS (you can only hide content), that is only possible via JavaScript and can possibly be achieved via an autoconfig.cfg file.

Is there any example how to use autoconfig.cfg to do this?

See threads like these:

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

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;