firefox image width fix

[spences10]

Referencing #419

Closes #706

-moz-document url-prefix() {
    /* Firefox doesn't respect max-width in certain situations */
    img { width: 100%; max-width: -moz-max-content; }
}

[spences10]

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[spences10]

SO answer: https://stackoverflow.com/a/33791838/1138354