tabcounter: show border in esr, make it less tiny

Switching to a better suited color-variable that also works with ESR, also increasing counter size to make it more readable.

I think I still prefer the previous font size, as it fit the tab close button and the new tab button better - I encourage playing with the font-size (and maybe padding) before merging.

src/userChrome/tabcounter.css

@@ -38,7 +38,7 @@
             left: 50%;
             transform: translateX(-50%);
             /* Tab Manager Menu button styling inspired by Firefox for Android */
-            border: 1px solid var(--tab-icon-overlay-fill);
+            border: 1px solid var(--toolbarbutton-icon-fill);
             border-radius: var(--tab-border-radius);
             padding: 0 5px 0 5px;
             font-size: 10px !important;