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.
Firefox does NOT and never has used Webkit. Like SeaMonkey, Galeon, Camino, Flock and several other browsers it is based on
Mozilla. Chrome, Opera, and Slepnir use
Blink (a fork of Webkit) while Microsoft Edge and Universal Windows Platform use
EdgeHTML.
Webkit is used in Safari, iCab 4, Omniweb, Android, and several Adobe products.
The compatibility issue in Safari is due to the fact webkit is deeply integrated into both MacOS and iOS and used not only by a variety of other Apple and third party apps but even the OS itself. Firefox does not need to concern itself with OS and third party apps compatibility.
