--- a/toolkit/components/extensions/ExtensionUtils.jsm
+++ b/toolkit/components/extensions/ExtensionUtils.jsm
@@ -23,20 +23,25 @@ function getConsole() {
   });
 }
 
 XPCOMUtils.defineLazyGetter(this, "console", getConsole);
 
 const appinfo = Cc["@mozilla.org/xre/app-info;1"].getService(Ci.nsIXULRuntime);
 
 let nextId = 0;
-const uniqueProcessID = String(appinfo.uniqueProcessID);
+const uniqueProcessID = appinfo.uniqueProcessID;
+// Store the process ID in a 16 bit field left shifted to end of a
+// double's mantissa.
+const processIDMask = uniqueProcessID * (2 ** 37);
 
 function getUniqueId() {
-  return `${nextId++}-${uniqueProcessID}`;
+  // Note: We can't use bitwise ops here, since they truncate to a 32 bit
+  // integer and we need all 52 mantissa bits.
+  return processIDMask + nextId++;
 }
 
 
 /**
  * An Error subclass for which complete error messages are always passed
  * to extensions, rather than being interpreted as an unknown error.
  */
 class ExtensionError extends Error {}