Be specific about vulnerabilities (plural).

Also, this has been published:
  https://www.mozilla.org/en-US/security/advisories/mfsa2020-13/

wiki/src/news/version_4.5.mdwn

@@ -26,8 +26,8 @@ starting from Tails.</p>
 
 - Update *Tor Browser* to [9.0.9](https://blog.torproject.org/new-release-tor-browser-909).
 
-  This update fixes a [[!mfsa 2020-11 desc="critical vulnerability"]] in
-  *Firefox*.
+  This update fixes several [[!mfsa 2020-13 desc="vulnerabilities"]] in
+  *Firefox*, including [[!mfsa 2020-11 desc="some critical ones"]].
 
   Mozilla is aware of targeted attacks in the wild abusing this
   vulnerability.