mitm-name in netError.dtd

Hi,

While going through the latest incomplete strings in Pontoon, I see
three strings in browser/chrome/overrides/netError.dtd refer to an
entity `mitm-name` via a `<span class='mitm-name'/>` element.

I haven't found any comments or further references to it, so I wonder
what this is going to be replaced with in the final string? I'd assume
MITM.

OTOH, I'm unsure if the acronym is considered to be something like
HTTP, and if not, some locales may want to use their own version.

Regards,
Julen.