Lets go straight to the point. First, to those you a newbie in editing userChrome.css, locate the file using this guide.
Then, just add these lines in userChrome.css.
toolbar#toolbar-menubar {display : none !important}Done.
.toolbarbutton-menubutton-button > .toolbarbutton-box,
.toolbarbutton-1 > .toolbarbutton-box
{
max-width: 16px !important;
text-align: center !important;
}
(I'm in lazy mode)
Edit: For the Menu button, use the Compact Menu extension.