The not-cleaned-up version of https://gist.github.com/joeytwiddle/6358913

userChrome.css

/*
 * 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 */

/* This surrounds the location bar and the bookmarks toolbar */
#navigator-toolbox {
    /* border: 1px solid black !important; */
}
/* Bookmarks Bar */
#PersonalToolbar {
    border: 1px !important;
}

/*Added to re-enable min tab width=0 in Firefox-4 */
.tabbrowser-tab:not([pinned]) {
    min-width: 1px !important;
}

/* Joey's styles for TreeStyleTabs */
/* These do make a difference, but a little opacity is still inherited, and the shadow still appears on the text (in Metal theme). */
/* Note that they will not appear in the inspector because they are not applied when using chrome://browser/content/browser.xul */
/* This one is probably not needed. */
.treestyletab-tabbar-toolbar {
    opacity: 1.0 !important;
}
/* This is the background which is visible when it is not filled with tabs (or when tabs are indented).  It also contains the actual tab labels. */
.tabbrowser-tabs[treestyletab-mode="vertical"] {
    opacity: 1.0 !important;
}
.tabbrowser-tabs[treestyletab-mode="vertical"] .tabbrowser-tab {
    opacity: 1.0 !important;
    text-shadow: 0px 0px 0px white !important;
    /* Use these for metal/sidebar themes, to close the large gaps between tabs.
    margin-top: -4px !important;
    margin-bottom: -4px !important;
    */
    /* Works but destroys the border's simple 3d effect. */
    /* For the simple theme, make tabs a little bit shorter. */
    margin-top:    -2px !important;
    margin-bottom: -2px !important;
    /* These do nothing */
    /*
    padding-top: 1px solid white !important;
    padding-bottom: 1px solid black !important;
    padding-top: -2px !important;
    padding-bottom: -2px !important;
    padding-top: 4px solid red !important;
    padding-bottom: 4px solid green !important;
    */
    /* Appears to do nothing: */
    /* background-color: #ff3300; */
}
/* Highlight the background of the current tab a bit more strongly, remove the borders since they don't work with the shrinking above. */
.tabbrowser-tabs[treestyletab-mode="vertical"] .tabbrowser-tab[selected='true'] {
    /* border-top: 1px solid red !important; */
    /* -moz-border-top-colors: ThreeDHighlight transparent !important; */
    /* -moz-border-top-colors: none; */
    -moz-border-top-colors: transparent !important;
    /* padding-top: 1px solid red !important; */
    /* background-color: rgb(96, 160, 238) !important; */
    background-color: none !important;
    background: -moz-linear-gradient(top, rgb(174,211,255), rgb(92,154,229)) repeat-x !important;
}
.tabbrowser-tabs[treestyletab-mode="vertical"] .tabbrowser-tab:not([selected='true']) {
    /* border-top: 1px solid black !important; */
    /* -moz-border-top-colors: ThreeDHighlight transparent !important; */
    -moz-border-top-colors: transparent !important;
    /* padding-top: 1px solid black !important; */
    /* background-color: none !important; */
    background: -moz-linear-gradient(top,  #dddddd,  #bbbbbb) repeat-x !important;
}
.tabbrowser-tabs[treestyletab-mode="vertical"] .tabbrowser-tab[selected='true']:hover {
    -moz-border-top-colors: !important;
}
.tabbrowser-tabs[treestyletab-mode="vertical"] .tabbrowser-tab:not([selected='true']):hover {
    -moz-border-top-colors: !important;
}
/* Set the favicon's background too */
.tabbrowser-tabs[treestyletab-mode="vertical"] .tabbrowser-tab[selected='true'] * {
    /* background-color: rgb(96, 160, 238) !important; */
    background-color: transparent !important;
    /* background: -moz-linear-gradient(top,  rgb(106,170,245),  rgb(77,122,177)) repeat-x !important; */
    background: transparent;
}
.tabbrowser-tabs[treestyletab-mode="vertical"] .tabbrowser-tab:not([selected='true']) * {
    background-color: transparent !important;
    /* background: -moz-linear-gradient(top,  #dddddd,  #bbbbbb) repeat-x !important; */
    background: transparent;
}

.tabbrowser-tabs[treestyletab-mode="vertical"] .tabbrowser-tab:not([pinned="true"]) {
    /* May be a bit faster? */
    /* Nope; ignored. */
    box-shadow: none;
}

.tabbrowser-tabs:not([treestyletab-tabbar-position="top"]) .tabbrowser-tab, .tabbrowser-tabs:not([treestyletab-tabbar-position="top"]) .tabbrowser-tab:not([selected="true"]) {
    /* Nope */
    /*
    border-top: 4px solid green;
    border-bottom: 4px solid red;
    */
}