The argument they're making (which I think is actually a fairly good one) is that this is to users benefit; the economic incentives for advertisers to circumvent anti-tracking tech are quite large. They claim that entirely preventing tracking is brittle; it's likely to be continual game of whack-a-mole that de facto will allow users to be tracked. Instead, giving advertisers a little more of what they actually want (better targeting) without the intermediate step of doing that by tracking users directly removes the incentive to pierce privacy.
Personally, I'm not a fan of advertising for reasons beyond privacy; it's also simply attention-sapping mental noise. Yet I'm not sure that's relevant here; removing some harm might be better than nothing even if I'm not happy with the end result.
The informed-consent angle seems fairly superficial. Yes, that matters - but the essential case is only if the tech is _not_ privacy preserving. Additionally, the ability to experiment is essential; to the extent this doesn't harm users and truly is small-scale I we should embrace experiments even if we don't personally at first glance embrace this specific one. It may seem polite to ask users for consent even here (and clearly that would have been the better PR choice), but I'm also not a fan of largely irrelevant consent forms, especially when asked prematurely - that's pretty close to spam. Sure, on Hacker News that may seem like an important question, but that's hardly a normal or representative slice of the population, and I'm just not sure I share the outrage here.
What exactly is the shadiness you're referring to here? Is it a lack of trust concerning their intentions? Is it a lack of trust that this is privacy preserving? Is it that you don't believe this experiment is small scale (enough or at all)? Or is that you don't trust they'll actually evaluate the experiment fairly, instead opting to push things through?
Another thing coloring my perspective here is that the open web sure seems to be heading in the wrong direction. Chromium is pretty dominant and none of the chromium derived browers appear to have resources (or willingness to spend them) to significantly depart from Chrome while still providing prompt security updates. Webkit is languishing and poorly updated in practice by users. De facto google seems to be pushing users towards at least pervasive tracking and leverage by google; Apple seems to support mostly those web features that don't risk competing with their app store. This isn't great; I'd love to see some competition.
Perhaps Firefox could somehow provide opt-in but noisy questions for users that want that, without harassing the likely large majority that doesn't see the point. It's a shame to chase away users over something like this, that's for sure.