Bug 1584273 - Disable the megabar animation when prefers-reduced-motion is true. r=dao

Differential Revision: https://phabricator.services.mozilla.com/D47348

--HG--
extra : moz-landing-system : lando

browser/themes/shared/urlbar-searchbar.inc.css

@@ -116,6 +116,13 @@
   transition-timing-function: var(--animation-easing-function);
 }
 
+@media (prefers-reduced-motion: reduce) {
+  #urlbar[breakout][breakout-extend-animate],
+  #urlbar[breakout][breakout-extend-animate] > #urlbar-input-container {
+    transition-duration: 0s;
+  }
+}
+
 #urlbar[breakout][breakout-extend] {
   /* The z-index needs to be big enough to trump other positioned UI pieces
      that we want to overlay. 3 is used in the tab bar. */