Motion Principles
Firefox motion is quick, supportive and whimsical.
Quick
Animations should be fast and nimble, never keeping the user waiting longer than is necessary. The aim is to prioritize user-perceived performance over technical benchmarks.
Supportive
Motion should help ease the user through the experience. It should aid the flow of actions, giving clear guidance for user orientation: spatial or temporal.
Whimsical
Animations should have a playful and delightful personality when appropriate.