userChrome.css for TreeStyleTabs. Kill transparency, don't shadow text, squeeze the tabs closer together. This addresses some issues I had with TreeStyleTabs: Too much transparency, shadowed text sucks, tabs are too widely spaced.

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 */

/* Joey's styles for TreeStyleTabs */
/* 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;
    */
    /* For the simple theme, make tabs a little bit shorter. */
    /* Works but destroys the border's simple 3d effect. */
    margin-top:    -2px !important;
    margin-bottom: -2px !important;
}
/* 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'] {
    -moz-border-top-colors: transparent !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']) {
    -moz-border-top-colors: transparent !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: transparent !important;
    background: transparent;
}
.tabbrowser-tabs[treestyletab-mode="vertical"] .tabbrowser-tab:not([selected='true']) * {
    background-color: transparent !important;
    background: transparent;
}

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

From here: piroor/treestyletab#551