As someone who spent a bit of time earlier this year (pre-rebrand) on firefish poking around on it I can personally vouch that the reviewer (@[email protected]) has definitely eaten the dogfood here and is pretty much spot on about everything.
I was pleasantly surprised to see clips described as your favourite feature ... mine too! One thing you didn't quite make clear is that they can be either private or public. A public "clip" (collection of bookmarks) is a great way of sharing a series of posts. In a way it's a nifty and very easy way of building up a web page of some sort. A nice example usage I ran into was creating a public clip of polls which together formed a survey that I could link to with a single URL.
Another thing you didn't seem to touch on is MFM and the properly crazy things you can do with it (here's a public clip of notable animations using MFM) but also the simple things that really do add some expression to your posts (colored text, actual bold and italic formatting, font size changes).
Longer posts may have been missed too? In my experience the 500 char limit of mastodon is just annoying. On firefish, with 4000 chars, you never have to worry about it ... just write and it'll be fine ... just about any thread on masto can fit in a single post on firefish.
You're absolutely right about Antennas ... biggest disappointment I've had on the fediverse ... seriously don't trust anyone who tells you they're awesome as they're just spouting hype without having actually tried to use the feature for anything but the most basic things that can be done elsewhere.
And yea, the instability of the main instance is a serious problem. It's the main reason I don't use firefish anymore. Not only was the stability annoying, but the admins, IMO, are not quite upfront about how much of an issue it is and why it's happening (there's a competency gap that isn't or wasn't being managed well enough). Which is a shame because there was a moment there where firefish really could have taken off more than it has but a number of people, myself included bounced off of it.
While you praise the UI, I think there's a fair critique around it being too much with too many features with text that is generally too small for many. Users have been asking since I was there for a more streamlined UI and I think many would enjoy that.
As you say, the Gallery feature could do with some love. Same goes for the Page feature which is a much more flexible tool for building effectively a web page except the UI is pretty bad and way too complex (an unaltered hangover from misskey I suspect).
In the end, what firefish/calckey/misskey shows is that the fediverse doesn't need to merely be either a twitter or instagram or reddit clone ... it can be many things often combined in different ways. Many have kinda forgotten (or worse never realised) that that can be the case largely because of how stagnant big social platforms are. If firefish gets its shit together, I agree with the reviewer, it could really take off.
Another more gossipy story around this is that some past maintainers forked firefish (over some sort of spat with the current lead dev and admin AFAICT). It's called iceshrimp and last I heard the blahaj zone instance, who've always run a downstream fork, were going to align with iceshrimp over firefish. I don't know where all of that is up to now, though iceshrimp is still going (they have a few servers and users: https://fedidb.org/software/iceshrimp), but it does emphasise I think that there's something non-ideal at the developer level of the platform that probably needs to stabilise.