Install it from https://addons.mozilla.org/fr/firefox/addon/no-transition/
Website's animations and transitions are often inappropriate, irritating and CPU intensive. This extension disable those effects and make the web feel fast again!
This Firefox add-on injects CSS into web pages to disable CSS3 animation.
Today when you open a menu it often "slide" for 0.2 second or links change color by a slow fading... Those effect are made with CSS3 animations/transition and can be heavy or reduce the website accessibility. With this extension transition are instantaneous.
This is in the roadmap: #5