RSS Subscribe seems to be broken in FF 57 with a web based Registered Content Handler
I use an instance of Tiny Tiny RSS to read my RSS feeds. It includes code to register as a web handler for RSS and when I go to Settings -> General -> Applications -> Web Feed I can see that my Tiny Tiny RSS instance is selected as the Use option. If I select application details I see "This web application is hosted at:" and it says "http://www.mydomain.com/public.php?op=subscribe&feed_url=%s" (quotes only for readability)
However, with Firefox 57 (possibly older but I only noticed it today) when I click the "Subscribe to this Page" button next to the address bar nothing happens. It used to "push" me over to my TT-RSS feed subscription page. This happens with ANY page that has an RSS feed available. I do not see any bugs relating to subscription errors in the tracker nor do I see anything by Googling but the only difference I know of is the Firefox upgrade.