Bug 1718220 - Try to sync-decode to hopefully fix some frequent flickering tests.

MANUAL PUSH: Hopefully trivially fixes some orange CLOSED TREE.

browser/base/content/tabbrowser-tab.js

@@ -18,7 +18,7 @@
           <stack class="tab-icon-stack">
             <hbox class="tab-throbber" layer="true"/>
             <hbox class="tab-icon-pending"/>
-            <html:img class="tab-icon-image" role="presentation"/>
+            <html:img class="tab-icon-image" role="presentation" decoding="sync" />
             <image class="tab-sharing-icon-overlay" role="presentation"/>
             <image class="tab-icon-overlay" role="presentation"/>
           </stack>