That feature (more) they've been getting all that negative press over for the past two days is an absolutely gigantic non-issue. Like most anti-Mozilla stories end up being.

The whole thing is an experimental feature intended to replace the current privacy nightmare that is cross-site tracking cookies. As-implemented it's a way for advertisers to figure out things like "How many people who went to our site and purchased this product saw this ad we placed on another site?", but done in such a way that neither the website with the ad, nor the website with the product, nor Mozilla itself knows what any one specific user was doing.

There are definitely things that can be said about this feature, like "Fuck ad companies, it should be off by default" (my personal take). But the feature itself has virtually no privacy consequences whatsoever for anybody, and Mozilla is at least trying to build a system that would legitimately improve the privacy situation on the internet created by companies like Google.