bug: Support when border-spacing returns a single value

This is mostly a workaround for a [potential bug introduced with Firefox 134](https://bugzilla.mozilla.org/show_bug.cgi?id=1941066)

Based on the spec, `getComputedStyle(el).borderSpacing` should always return 2 values, even when they are identical.

On Firefox 134, it returns a single value when they are the same.

addon/src/utils/css-calculation.ts

@@ -8,7 +8,9 @@
 */
 export function getBorderSpacing(el: Element) {
   const css = getComputedStyle(el).borderSpacing; // '0px 0px'
-  const [horizontal, vertical] = css.split(' ');
+
+  const [horizontal, initialVertical] = css.split(' ');
+  const vertical = initialVertical === undefined ? horizontal : initialVertical;
 
   return {
     horizontal: parseFloat(horizontal ?? ''),