Try this code in userChrome.css

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

#PlacesToolbarItems .bookmark-item > .toolbarbutton-icon {display:none!important}

Unfortunately don't work, the empty space still present on the left.

Does other code in the userChrome.css work?

Sounds like one of the cases that you created a text file with a hidden .txt file extension.

What is the file type if you check this in the Properties via the right-click context menu?

Try to rename the current file with quotes around the name or use Notepad to create a new file and make sure to select All files when using Save as to save the file. Windows can be quite stubborn with adding a .txt file extension.

See also:

• http://kb.mozillazine.org/Editing_configuration/Troubleshooting

This is my current userChrome.css file:

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

/* Bookmark This Page */
#appmenu_bookmarkThisPage,
#menu_bookmarkThisPage,
#BMB_bookmarkThisPage { display: none !important; }

/* Subscribe to This Page */
#appmenu_subscribeToPage,
#appmenu_subscribeToPageMenu,
#appmenu_subscribeToPageMenu + menuseparator,
#subscribeToPageMenuitem,
#subscribeToPageMenupopup,
#menu_bookmarkAllTabs, /* show-only-for-keyboard */
#menu_bookmarkAllTabs + menuseparator,
#BMB_subscribeToPageMenuitem,
#BMB_subscribeToPageMenupopup,
#BMB_subscribeToPageMenupopup + menuseparator { display: none !important; }

/* Bookmarks Toolbar */
#appmenu_bookmarksToolbar,
#appmenu_bookmarksToolbar + menuseparator,
#bookmarksToolbarFolderMenu,
#bookmarksToolbarFolderMenu + menuseparator,
#BMB_bookmarksToolbar,
#BMB_bookmarksToolbar + menuseparator { display: none !important; }

/* Unsorted Bookmarks */
#appmenu_bookmarksPopup > menuseparator[builder="end"], #appmenu_unsortedBookmarks,
#bookmarksMenuPopup > menuseparator[builder="end"], #menu_unsortedBookmarks,
#BMB_bookmarksPopup > menuseparator[builder="end"], #BMB_unsortedBookmarks { display: none !important; }

/* Open all tabs */
.bookmarks-actions-menuseparator,
.openintabs-menuitem { display: none !important; }

/* Bookmarks Menu - Hide favicons and placeholders but show folder icons for menus */
/* #bookmarksMenu menuitem .menu-iconic-icon {display:none !important;} */

I use some optimizations for the bookmark toolbar, like hide some options i don't need. But i don't think that this interfere.

The last one is diable indeed till i find a way to align the bookmark , if this is possible.

You wrote in the first post: remove the empty favions placeholders in the bookmark toolbar
So that looks like it meant for the Bookmarks Toolbar and the CSS I posted above should hide the favicons on that Bookmarks Toolbar.
For icons on the tab bar you would need different code.

Code with a #bookmarksMenu selector will work for the Bookmarks menu on the menu bar when this toolbar is visible.

Did you test this with the default theme and possibly other extensions disabled in case you have some that affect the Bookmarks Toolbar?

Yes, indeed i try to disable all my extensions. Have you any other tip to try?

Does the code I wrote above remove the favicons on the Bookmarks Toolbar?

Just a comment in passing

Why are you trying to remove favicons blank icon Normally you probably should not have many. There would often be a correct icon there.

I suppose I am wondering are you trying to remove all the favicons or just blank ones, and do you have a lot of blank ones for some reason ?

Maybe is my fault not to be more clear, i post an image below so you can understand better what i want to do if this is possible:

http://oi39.tinypic.com/209sxgl.jpg

On the left column there is the normal bookmar menù without any code, in the right the bookmark menù with the code, as you can notice i remove the placeholder icon of the favicons but there is now a blank space, i want to move the text covering this blank space to be aligned vertically with the yellow folders. Is this possible or not?

Is not a big deal in the end, just wondering.