On FF56 the RSS button in the location bar keeps disappearing for me,
I have two other addons that do anything to the location bar, but I used them both in parallel with CTR for years. (The addons being Flagfox that places a country flag, and Location Bar Enhancer, which adds a scrollbar background and breadcrumbs for the url. (This needed an unofficial fix))
It comes to mind that there was a CTR update a few days ago, maybe it breaks during update?
Edit2:Never mind this. It looks like a plain restart is enough for it to disappear...