Bug 1610614 - Part 3: Check for gFissionBrowser in dragover handler, r=mconley

Differential Revision: https://phabricator.services.mozilla.com/D61925

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

browser/base/content/tabbrowser-tabs.js

@@ -1900,6 +1900,12 @@
             return "none";
           }
 
+          if (
+            window.gFissionBrowser != sourceNode.ownerGlobal.gFissionBrowser
+          ) {
+            return "none";
+          }
+
           return dt.dropEffect == "copy" ? "copy" : "move";
         }
       }