This is a follow-up bug of Bug 1322780. We support unprefixed max-content, so it's worth to investigate if some of these tests  that used -moz-max-content can be moved to the w3c-css reftest directory now.  https://searchfox.org/mozilla-central/rev/6ddb5fb144993fb5de044e2e8d900d7643b98a4d/layout/reftests/flexbox/reftest.list#3-4
Thanks for following up on this! Yeah, I think we do have some flexbox tests where the only reason they're in our private directory is due to their usage of these prefixed keywords.
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