24.6 ESR from June 2014. The side navs are now on top of the main content. So the main contend slides under the side nav and is hard to read. I understand that it's a 3 year old browser, but it's on a Win7 machine, like lots of other web users.
I'd update!
1. https://www.cvedetails.com/vulnerability-list/vendor_id-452/...
Maybe older Firefox has a bug in calc(), disabling that property in a newer browser is enough to shift the content below the sidenav because it falls back to width: 100%;. I see what you mean about hard to read, if I disable the calc() width, the sidenav stays put but the main content scrolls on top of it. That's a little confusing because the sidenav staying put is due to the property position: sticky; but caniuse.com says Firefox didn't support that until version 26, version 32 without having to enable a flag.
Anyhow, I do think government guidelines in particular should handle older browsers better than this page is demonstrating. Part of the point of using frameworks for libraries and others is they can maintain and aggregate all the little solutions to problems between browser implementations and between older and newer browsers.