Between the huge and complex attack surface and being exposed to a huge number of untrusted websites, running a browser without security updates is pretty risky. So I'd call any unsupported browser "severely outdated".

Long term support (ESR) Firefox releases are supported for about 15 months from release. And even that means using a major version that old, not a point version that old. Firefox 57 wasn't even an ESR, so it went out of support a couple of months after release.