add Firefox 81 flex-direction fully supported (#13736)
add Firefox 81 flex-direction fully supported (#13736)

Fixes https://github.com/mdn/browser-compat-data/issues/13735
mdn/browser-compat-data,chrisdavidmills/browser-compat-data,mdn/browser-compat-data,chrisdavidmills/browser-compat-data