bug 309902: This was supposed to land with the fix for bug bug 287990, but I forgot to check it in then, checking it in now (changes had r=jst sr=dbaron).

layout/style/html.css

@@ -153,7 +153,7 @@ listing {
   margin: 1em 0;
 }
 
-xmp, pre {
+xmp, pre, plaintext {
   display: block;
   font-family: -moz-fixed;
   white-space: pre;