Download it at https://mdn-downloads.s3-us-west-2.amazonaws.com/developer.mozilla.org.tar.gz
The “Downloading content” section of About MDN provides the above link (for a tarball download) along with guidance on other ways to access the MDN content, both as single pages and via third-party tools.
And others : don’t mirror with wget & co, this is putting un-needed pressure on the website and hinders other users. At least make sure https://developer.mozilla.org/robots.txt (which asks for gentle throttling) is properly handled. Wget does not handle this for instance (http://savannah.gnu.org/bugs/?30999).