sometimes it's nice to see what articles the feed contains in before deciding whether to subscribe and since the feed preview function was removed by the geniuses at mozilla, it would be great to have this back and would complete the feature set i think (and avoids having to install yet another extension)

if this gets added, i would suggest changing the behavior of the feed detection icon to where a primary/left click open a menu containing the available feeds - after clicking one, the preview is shown with a 'Subscribe' label/icon/link

rsspreview seems to do a pretty good job and maybe could provide sample code

ps: the 2.18 update was quite impressive - thanks much for your work!