Trying Feedbro while Brief updates to work on FF57
There's a lot of small issues here and there: lack of support for Live bookmarks (which Firefox has been using for some years now), UI unpredictability and general weirdness (Not only can't I set an exact number of minutes for scan interval, you chose to represent a year as 525600 minutes!), can't drag'n'drop links for fast feed creation, can't select multiple feeds... Mostly minor UI stuff that builds up and makes the add-on annoying to deal with, but It seems likely that they'll get fixed over time.

What really stands out is the rules system. I've got a feed that's always giving me trouble, both here and back on Brief, where marking the "don't mark updated articles as unread" checkbox solved it's hijinks, but since Feedbro lacks that I tried to solve it with rules. The hijinks this time is that every time the scan interval completes, a single, old article reappears as brand new, and my gut feeling was to use the rules to compare if the time at which it was detected was more recent than the earliest article. Sadly, you can't compare articles, nor check any kind of time atribute. So as a emporary measure I've placed a rule to mark all updates from said feed as read, and not notify me of said updates. Bummer.
Then there's another feed that usually posts multiple articles at once, to be read in a particular order. Since notifications are per article, when the update hits I get spammed with notifications, and was about to set a rule to check wetherarticles have been posted within seconds of each other, only to remember I can't check that. Bummer again.
Those are just two specific cases, what Im really trying to get at is that the rules system has the potential to become an incredibly powerful tool.

TL;DR: Feedbro really shows potential to become great, but right now it's just fine and usable. Won't be using it yet, but will definitely come back to check again in a few months to see what's changed.