--- a/toolkit/mozapps/extensions/internal/XPIProvider.jsm
+++ b/toolkit/mozapps/extensions/internal/XPIProvider.jsm
@@ -731,16 +731,19 @@ function canRunInSafeMode(aAddon) {
  */
 function isUsableAddon(aAddon) {
   // Hack to ensure the default theme is always usable
   if (aAddon.type == "theme" && aAddon.internalName == XPIProvider.defaultSkin)
     return true;
 
   if (mustSign(aAddon.type) && !aAddon.isCorrectlySigned) {
     logger.warn(`Add-on ${aAddon.id} is not correctly signed.`);
+    if (Preferences.get(PREF_XPI_SIGNATURES_DEV_ROOT, false)) {
+      logger.warn(`Preference ${PREF_XPI_SIGNATURES_DEV_ROOT} is set.`);
+    }
     return false;
   }
 
   if (aAddon.blocklistState == Blocklist.STATE_BLOCKED) {
     logger.warn(`Add-on ${aAddon.id} is blocklisted.`);
     return false;
   }