diff --git a/testing/profiles/prefs_general.js b/testing/profiles/prefs_general.js
index 24f67b4b68b4..9b009270a381 100644
--- a/testing/profiles/prefs_general.js
+++ b/testing/profiles/prefs_general.js
@@ -357,3 +357,7 @@ user_pref("browser.usedOnWindows10.introURL", "");
 // For Firefox 52 only, ESR will support non-Flash plugins while release will
 // not, so we keep testing the non-Flash pathways
 user_pref("plugin.load_flash_only", false);
+
+// Don't block old libavcodec libraries when testing, because our test systems
+// cannot easily be upgraded.
+user_pref("media.libavcodec.allow-obsolete", true);