They completely agree how to proceed, Firefox and Chrome both moved to using feature flags instead of vendor prefixes a while back, but there are so many -webkit- prefixed "advanced" properties in the wild, and developers only testing their site in Chrome (which hasn't removed the old prefixed properties for compatibility reasons) that Mozilla felt compelled to add support for these. Incidentally, they're following Opera[0] and Microsoft[1] in doing so.
> while Apple announces dropping the same prefixes
Apple didn't announce dropping support for existing prefixed properties (at best they noted that they may remove some over time depending on compatibility needs), only that new features will be gated instead of prefixed going forwards.
> it also means they'll have this prefix code as an artifact for some time while it becomes increasingly irrelevant.
Prefixed properties are just specific names, the vendor prefix is incidental, Mozilla implemented and enabled a bunch of properties named -webkit-{something} instead of {something}.
[0] http://www.sitepoint.com/opera-css3-webkit-prefix/
[1] https://msdn.microsoft.com/en-us/library/mt270097%28v=vs.85%...