Reduce Firefox interface size, fix minor annoyances. Some tags might not do much, but the latest userChrome specification I found was from circa 2007, and restarting Firefox to check changes becomes a bother from the 5th time onwards...

userChrome.css

/*
 * Edit this file and copy it as userChrome.css into your
 * profile-directory/chrome/
 */

/*
 * This file can be used to customize the look of Mozilla's user interface
 * You should consider using !important on rules which you want to
 * override default settings.
 */

/*
 * Do not remove the @namespace line -- it's required for correct functioning
 */
@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* set default namespace to XUL */


/*
 * Some possible accessibility enhancements:
 */
/*
 * Make all the default font sizes 20 pt:
 *
 * * {
 *   font-size: 20pt !important
 * }
 */
/*
 * Make menu items in particular 15 pt instead of the default size:
 *
 * menupopup > * {
 *   font-size: 15pt !important
 * }
 */
/*
 * Give the Location (URL) Bar a fixed-width font
 *
 * #urlbar {
 *    font-family: monospace !important;
 * }
 */

/*
 * For more examples see http://www.mozilla.org/unix/customizing.html
 */

#urlbar {
    font-family: monospace !important;
    font-size: 80% !important;
    letter-spacing: 90% !important;
}

statuspanel {
    max-width: 100% !important;
}

menubar > menu {
    font-size: 9px !important;
}

.tab-text {
    font-size: 9px !important;
}

.menu-text {
    font-size: 9px !important;
}

.toolbarbutton-text {
    font-size: 9px !important;
}

.statusbarpanel-text {
    font-size : 9px !important;
    font-family: monospace !important;
}

.tabbrowser-tab {
    height: 20px !important;
    min-height: 20px !important;
    max-height: 20px !important;
    margin: 0x;
}
.tabbrowser-tabs {
    height: 100% !important;
}
.tab-image-right, .tab-image-left, .tab-image-middle {
    min-height: 16px !important;
    max-height: 16px !important;
}
.tab-icon-image, .tab-close-button {
    /* margin: 0px !important; */
    height: 16px !important;
    width: 16px !important;
}
.tab-content {
    margin-top: -8px;
}
.tab-text {
    margin-top: 1px !important;
}
.tab-close-button > .toolbarbutton-icon {
    margin-top: 1px !important;
}
.tabbrowser-arrowscrollbox > .scrollbutton-up,
.tabbrowser-arrowscrollbox .scrollbutton-down,
.tabbrowser-arrowscrollbox .scrollbutton-down-box,
.tabbrowser-tabs .tabs-alltabs-box,
.tabbrowser-tabs .tabs-alltabs-box-animate,
.tabbrowser-tabs .tabs-alltabs-button {
    height: 18px !important;
    margin: 1px !important;
}
.tabbar-newtab-button {
    height: 20px !important;
}

/* remove padding on toolbar */
#navigator-toolbox {
    padding: 0px !important;
    height: 25px !important;
    padding-top: -3px !important;
    /* background-color: #f00 !important; */
}

/* minifi nav bar */
#nav-bar {
    height: 20px !important;
    padding: 0px !important;
}

/* a button in the toolbar */
.toolbarbutton-menubutton-button {
}

.toolbarbutton-icon {
    width: 16px !important;
    height: 16px !important;
}

/* minifi addon bar */
#addon-bar, #addon-bar > * {
    height: 20px !important;
}

/* addon bar (on bottom) */
#addon-bar {
    width: 20px !important;
}


/* .toolbar { */
/*     height: 90% !important; */
/* } */