Add -moz-box-sizing rules to the styles that use box-sizing

Apparenly Firefox still hasn't caught up on the unprefixed version.

addon/merge/merge.css

@@ -28,6 +28,7 @@
   z-index: 2;
   display: inline-block;
   height: 100%;
+  -moz-box-sizing: border-box;
   box-sizing: border-box;
   overflow: hidden;
   border-left: 1px solid #ddd;

addon/tern/tern.css

@@ -14,6 +14,7 @@
   line-height: 16px;
   text-align: center;
   color: white;
+  -moz-box-sizing: border-box;
   box-sizing: border-box;
 }
 .CodeMirror-Tern-completion-unknown:before {