$ portgraph -p www/firefox
$ egrep -e '-> "lang/python27"' firefox
"graphics/mesa-libs" -> "lang/python27" [color="#009999"]
"www/node" -> "lang/python27" [color="#009999"]
"lang/spidermonkey60" -> "lang/python27" [color="#009999"]
"www/firefox" -> "lang/python27" [color="#009999"]
poudriere testport
with a modified Makefile.common to build the package using python3.5+ per the Mesa 3D compilation instructions instead of python2.7, and it built fine. So there's theoretically no reason to require Python 2.7 for graphics/mesa-libs and graphics/mesa-dri if the builds don't fail or otherwise cause trouble on other architectures.