diff --git a/build/moz.configure/warnings.configure b/build/moz.configure/warnings.configure
index 8e1689e10343..0f8047bc8041 100644
--- a/build/moz.configure/warnings.configure
+++ b/build/moz.configure/warnings.configure
@@ -311,6 +311,10 @@ check_and_add_warning("-Wno-vla-cxx-extension", cxx_compiler)
 # Avoid requiring complicated logic for extra warning flags in moz.build files.
 check_and_add_warning("-Wno-unknown-warning-option")
 
+# clang 21 warns about charN_t conversions, and we have a lot that need a careful
+# audit rather than plainly adding static_casts.
+check_and_add_warning("-Wno-character-conversion")
+
 set_config("WARNINGS_CFLAGS", warnings_flags.cflags)
 set_config("WARNINGS_CXXFLAGS", warnings_flags.cxxflags)
 set_config("WARNINGS_HOST_CFLAGS", warnings_flags.host_cflags)