Firefox

userChrome.css

Path:

~/.mozilla/firefox/USER/chrome/userChrome.css

Default:

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* set default namespace to XUL */

Tree Style Tab

tab:not([selected="true"]) {
    background-color: #B9B9BD !important;
    color: #000000 !important;
}