Fix sticky header issue in Firefox (github#27159)

* Redundant sticky?

* update zindex to bring scroll button up

Co-authored-by: Grace Park <[email protected]>

Author: rsese

components/DefaultLayout.tsx

@@ -100,7 +100,7 @@ export const DefaultLayout = (props: Props) => {
           <SupportSection />
           <SmallFooter />
           <ScrollButton
-            className="position-fixed bottom-0 mb-4 right-0 mr-4"
+            className="position-fixed bottom-0 mb-4 right-0 mr-4 z-1"
             ariaLabel={t('scroll_to_top')}
           />
         </footer>

components/page-header/Header.tsx

@@ -52,7 +52,7 @@ export const Header = () => {
   return (
     <div
       className={cx(
-        'border-bottom d-unset color-border-muted no-print z-3 color-bg-default position-sticky top-0',
+        'border-bottom d-unset color-border-muted no-print z-3 color-bg-default',
         styles.header
       )}
     >