Index: thunderbird-128.4.2/comm/mailnews/base/src/MboxMsgInputStream.cpp
===================================================================
--- thunderbird-128.4.2.orig/comm/mailnews/base/src/MboxMsgInputStream.cpp
+++ thunderbird-128.4.2/comm/mailnews/base/src/MboxMsgInputStream.cpp
@@ -263,6 +263,7 @@ class MboxParser {
       default:
         MOZ_ASSERT_UNREACHABLE();  // should not happen
     }
+    return data;
   }
 
   // Attempt to parse a "From " line to extract sender and timestamp.