We introduced unprefixed versions of transforms in bug 745523; at some point in the future we'll need to remove support for the prefixed variants.
why? wouldn't it be easy to support the alias indefinitely? ..damn interpreted language ways....
It wouldn't be too hard. However, it would encourage people to use the prefixed version, which means that their sites would only work in Gecko-based browsers. Mozilla's goal is to advance the open web, not to encourage a Gecko-specific web, so we want authors to use standard features, rather than Gecko-proprietary ones.
I didn't think about how it promotes using the un-prefixed version until now. Interesting.
aha, avoid the cargo cult cut-and-pastes from breaking
We could implement a pref first, like in bug 804944.
Posted the site compatibility doc: https://www.fxsitecompat.com/en-US/docs/2015/prefixed-css-animations-transforms-transitions-support-will-be-removed/