I use the Awesome RSS addon [1] to get the icon in the URL bar back. Also RSSPreview [2] to see whether the feed actually contains what I want to subscribe to. The previous integration where you could add the feed to Thunderbird directly from Firefox with a single click was much better, though.
[1] https://addons.mozilla.org/en-US/firefox/addon/awesome-rss/
[2] https://addons.mozilla.org/en-US/firefox/addon/rsspreview/