diff --git a/mozilla/layout/mathml/content/src/mathml.css b/mozilla/layout/mathml/content/src/mathml.css
index 8547d1d8b21..ef1e461c783 100644
--- a/mozilla/layout/mathml/content/src/mathml.css
+++ b/mozilla/layout/mathml/content/src/mathml.css
@@ -53,6 +53,7 @@ math {
   font-size: inherit;
   font-style: normal;
   font-family: CMSY10, Symbol, Times, Lucida Sans Unicode, MT Extra, Math1, Math2, Math3, Math4, Math5, serif;
+  -moz-float-edge: margin-box;
 }
 math[mode="display"], math[display="block"] {
   display: block;