(I didn't know of this company's existence until their non-existence today)
I don't think the idea is really that bad, and a form-filling browser extension would not be comparable to what they were trying to do here. The idea is pretty compelling in isolation - your random mom n pop ecommerce site can compete with Amazon on price for a lot of things, but it can't compete with convenience.
If I go to Amazon I can buy something with one or two clicks and be pretty certain that my card details aren't going to get stolen and the thing I ordered will probably arrive when they say it will just about.
If I go to a random ecommerce site I probably have to register, have to enter my card details and confront the possibility of getting ripped off. It's so much less convenient, so for most purchases I just go to Amazon, which is bad for a bunch of reasons.
What Fast was promising to do was to bring that convenience and trust to ecommerce sites, so that they could become more competitive with the Amazons of this world, get more sales, make more money, consumers save money, everyone wins.
Of course the problem is that they're only compelling when they're ubiquitous, and when everyone already has an account and everyone already trusts them. It's a bootstrapping problem, one that only really works if you burn a whole bunch of capital to get both consumers and suppliers signed up, or if you can piggyback off someone else's market penetration, e.g. Stripe, and even with that leg-up they weren't able to survive. All of this also ignores the existence of paypal, apple pay, google pay etc which do offer most of the convenience already.