The current version of Safari is supported by OS X/macOS 10.11, .12, & .13, two fewer OS versions than support the current version of Firefox, so the difference between the two isn't all that big, but I've got no idea what, if anything, NECESSITATES there being such a difference in the first place, since Safari and Firefox are both built on WebKit.

Perhaps someone with knowledge of the details can fill us in.

The new Great Equalizer is the SEND button.

In Memory of Harv: Those who can make you believe absurdities can make you commit atrocities. ~Voltaire