Bug 298699 Theme Manager's width is no longer fixed

patch by rob_strong@exchangecode.com r=bsmedberg a=bsmedberg

toolkit/mozapps/extensions/content/extensions.js

@@ -211,6 +211,7 @@ function Startup()
     var themePreviewArea = document.getElementById("themePreviewArea");
     themePreviewArea.hidden = false;
     gExtensionsView.removeAttribute("flex");
+    gExtensionsView.style.width = "20em";
   }
   
   // Set Initial Size