This small Style will enable one to change the Font Color , and Centre Text in the Firefox Tab
I use in conjunction with this style, which alters many of Firefox's Internal Fonts including Tabs
Please alter color to suit.
@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");
.tab-text
{
text-align: center !important;
color:#CCCCCC !important;
}