CSS

[LoolyTunes]

I think there should be a guide somewhere on how to create a userChrome.css file and then use it, it'll be really helpful to new users if they want to remove buttons or change icons when there's no feature to do that.

[Agentvirtuel]

Hello

For information, for illustrative purposes, but it is not always easy to find the required information
https://connect.mozilla.org/t5/ideas/ability-to-edit-context-menus-remove-unnecessary-options/idc-p/...

[siffemcon · Accepted solution]

There won't be an official guide because Mozilla does not officially support adding custom style rules.

[Agentvirtuel]

Hello

Additional information.

Open source, is cool, because is fun.

https://connect.mozilla.org/t5/ideas/allow-moving-the-hamburger-icon-to-the-left/idc-p/102470 
https://firefox-source-docs.mozilla.org/devtools-user/browser_toolbox/index.html

Other help https://www.reddit.com/r/FirefoxCSS 

[Agentvirtuel]

Hello

About, allow moving the hamburger icon, for information purposes.

Take a look at https://www.camp-firefox.de/forum/thema/139663-hamburger-menü-nach-links/?postID=1276224#post1276224

//PanelUI-menu-button verschiebbar.uc.js

(function() {
  if (location != 'chrome://browser/content/browser.xhtml')
    return;  
  var menuButton = document.getElementById('PanelUI-menu-button');
 
  if (!CustomizableUI.getPlacementOfWidget('ucjs_PanelUI-button')) { 
    try {
      CustomizableUI.createWidget({
        id: 'ucjs_PanelUI-button',
        type: 'custom',
        defaultArea: CustomizableUI.AREA_NAVBAR,
        onBuild: function(aDocument) {
          var toolbaritem = aDocument.createElementNS('http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul', 'toolbaritem');
          toolbaritem.id = 'ucjs_PanelUI-button';
          toolbaritem.className = 'chromeclass-toolbar-additional';          
          toolbaritem.setAttribute('label', 'Firefox Anpassen');          
          return toolbaritem;            
        }
      });
      if (!CustomizableUI.getPlacementOfWidget('ucjs_PanelUI-button')) {     
        menuButton.style.display = 'none';
        return; 
      };        
    } catch(e) {        
      menuButton.style.display = 'none';
      return;      
    };    
  };
  
  setTimeout(function() { 
   document.getElementById('ucjs_PanelUI-button').appendChild(menuButton);       
   menuButton.setAttribute('consumeanchor', 'ucjs_PanelUI-button');
   menuButton.setAttribute('label', 'Firefox Anpassen');
   menuButton.setAttribute('tooltiptext', 'Firefox Anpassen + Panel');        
  }, 0);

}());

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PanelUI-menu-button verschiebbar.uc.js

Take a look at
https://mozhelp.ddnss.de/dateien/forum/ff-vorbereitung-skripte.mp4
https://github.com/Endor8/userChrome.js/tree/master/userChrome#hinweis download and unpack the ZIP file

By way of illustration https://www.youtube.com/embed/9EqZ7L-8p_g