--- a/browser/themes/shared/tabs.inc.css
+++ b/browser/themes/shared/tabs.inc.css
@@ -181,20 +181,22 @@ tabbrowser {
   opacity: 0.7;
 }
 
 :root[sessionrestored] .tab-throbber[busy]:-moz-locale-dir(rtl)::before {
   animation-name: tab-throbber-animation-rtl;
 }
 
 @keyframes tab-throbber-animation {
+  0% { transform: translateX(0); }
   100% { transform: translateX(-100%); }
 }
 
 @keyframes tab-throbber-animation-rtl {
+  0% { transform: translateX(0); }
   100% { transform: translateX(100%); }
 }
 
 #TabsToolbar[brighttext] .tab-throbber[busy]:not([selected=true])::before {
   /* Don't change the --tab-loading-fill variable because this should only affect
      tabs that are not visually selected. */
   fill: #717171;
   opacity: 1;