create tab_closing_animation.css

chrome/tab_closing_animation.css

@@ -0,0 +1,19 @@
+/* Source file https://github.com/MrOtherGuy/firefox-csshacks/tree/master/chrome/tab_closing_animation.css made available under Mozilla Public License v. 2.0
+See the above repository for updates as well as full license text. */
+
+/* This style simply makes the tab stay visible while it is being closed as well as makes the transition a bit slower */
+
+.tabbrowser-tab:not([pinned],[fadein]){
+  transition-duration: 240ms, 240ms !important;
+  transition-timing-function: ease-out, ease-out !important;
+  visibility: visible !important;
+}
+.tab-content:not([fadein],[pinned])
+.tab-icon-pending:not([fadein]),
+.tab-icon-image:not([fadein]),
+.tab-label:not([fadein]){
+  visibility: visible !important;
+}
+.tab-label:not([fadein]){
+  display: flex !important;
+}