I've searched long enough for this to put it here for other searchers (especially concerning spacing or margins). If you want the height of your tabs to be a bit smaller (including text of course), you can do that by editing your "C:\Users\[YourUSername]\AppData\Local\Mozilla\Firefox\Profiles\[Your profile]\chrome\userChrome.css".  Create one from "userChrome-example.css" if it doesn't exist.
Here is the code you should add after "@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul")":
.tabbrowser-tabs {
    height: 18px !important;
}
.tabbrowser-tabs .tab-text {
    font-size: 80%;
}
.tabbrowser-tab
{
    margin-top:0 !important;
}
.tabbrowser-tabs tab label
{
    margin-top:-2px !important;
    height: 16px !important;
}