On the topic of XFMG, it is correct that we don't have any RSS feeds there anymore.
On the topic of Firefox, that seems to be a change of behaviour on their side. They used to have a built-in RSS reader but it has since been removed. You would need a browser add-on to replace the functionality.
That's a shame, RSS feeds from the media gallery can be useful as you can plug that into a system like https://dlvrit.com/ to automate the posting of media content from your forum to sites like twitter with accompanying hashtags. This creates a setup and forget advertising tool where users on other social media sites have a chance to discover your content and then visit your forum.