Here is how I changed default tab icons in Komodo Edit 7.1:
.tab-icon {
display: none;
}
.tab-image-left {
background-image: url(chrome://famfamfamsilk/skin/icons/page.png) !important;
width: 16px !important;
height: 16px !important;
}
#tabbed-view tabs > tab[label$=".js"] .tab-image-left {
background-image: url(chrome://famfamfamsilk/skin/icons/page_white_code.png) !important;
}
This code should go into userChrome.css