In -r39124, some additional issues were introduced:
* The 'overflow: hidden' on the `.site-header;` was added to prevent a side-scrolling issue, but it ended up cutting off the menu. It looks like the cause of the side-scroll was related to how the scroll down icon was rotated, so it's addressed by rotating the icon and not the `<a>`.
* Also, `min-height` doesn't play well with `display: table;` in Firefox. So it's addressed with just `height`.
Props laurelfulford, sami.keijonen.
See #38543.
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