Instead of proper format, it causes the problem in firefox. To fix the format, solve your problem.
Read this article: Firefox tries to download my RSS feed
RSS Feeds have a content type problem. Most people end up serving them with the content-type: text/XML. But this practice is frowned upon for several reasons. The main reason people don’t like text/XML is because of its very vague, there are content types such as application/rss+xml, application/rdf+xml, and application/atom+xml that describe the content of your feed much better than text/XML does. We should be using these types for our feeds.
The problem, however with the more descriptive content types is that Firefox and IE prompt you to download the XML file instead of displaying it in the browser like it would a text/XML document. More details read this
You have found some add-ons or extensions for firefox, chrome, opera, safari etc to validate your RSS feed
You can also use online validator tools to validate your RSS or ATOM