Although this is just my day long impression of it, this works as many people migrating from the removal of Firefox RSS would expect:
-Successfully loads the preview pages upon clicking on either the icon as it appears, or the RSS icons on the sites. You can also set the options to skip the preview and just save the RSS bookmark.
-Successfully displays/previews RSS from certain sites that would otherwise have an HTML error page, and save the RSS feed. The preview page itself is different.
-Successfully creates the RSS in bookmarks for you to access like before. You can also set in options what folder in bookmarks these folders appear in, and it's main by default. (Make sure you set this before you import)!
-Successfully imports OPML files (file that contain data on your old RSS bookmarks) and have those creted in the bookmarks.
-Many people would be glad to know the RSS icon appears in the address bar like older times, and the icon is hidden and doesn't take up space in the bar unless there is an RSS feed on the page (although I personally prefered the latter iteration where it was a customisable movable icon out of the address bar that faded out when there wasn't an RSS feed).
-Customisable features in terms of rates of saving bookmarks, how it displays they've been read and a few others.
There could be a distinction for folders having a unique icon to show they're RSS feeds, but this is pretty minor compared to how substantial this fix is.
-Apparently there are issues with usage and functionality on certain mobile devices for people. The ability to send an error log could be useful.
-The icon is strictly in black, which makes it hard to see in Mozzilla Dark theme,
Amazing job with this app, and thank you so much for making this.