diff --git a/configure.in b/configure.in
index 3752ec75601..9b8236d35c8 100644
--- a/configure.in
+++ b/configure.in
@@ -3118,12 +3118,15 @@ MOZ_ARG_ENABLE_BOOL(ldap-experimental,
     MOZ_LDAP_XPCOM_EXPERIMENTAL=1)
 
 dnl ========================================================
-dnl MATHML
+dnl MathML on by default
 dnl ========================================================
-MOZ_ARG_ENABLE_BOOL(mathml,
-[  --enable-mathml         Enable MathML ],
-    MOZ_MATHML=1
-    AC_DEFINE(MOZ_MATHML))
+MOZ_MATHML=1
+MOZ_ARG_DISABLE_BOOL(mathml,
+[  --disable-mathml      Disable MathML],
+    MOZ_MATHML= )
+if test "$MOZ_MATHML"; then
+  AC_DEFINE(MOZ_MATHML)
+fi
 
 dnl ========================================================
 dnl SVG