My own anecdotal evidence is completely the opposite. I've never heard anything but praise. Being able to open pdfs right in my browser with no stupid plugins is downright heavenly. There's only a couple PDFs I've ever seen that made pdf.js choke. I absolutely love it and am very glad I don't have to mess with some external PDF viewer or annoying plug-in ever again.
For the ones that do fail it's easy enough to download them and open them up in a different viewer.
("Initially flaky software improves over time" is a common story, albeit a boring one, and bad first impressions can be hard to shift.)
I'm always happy when it fires up and saves me having to switch to an external PDF viewer.
[citation needed]. remember that people bitching on web forums does not equal the general population's impression. firefox has extensive telemetry in the browser -- i'm certain that if australis hurt usage metrics, there'd be changes, or at least discussion in the mozilla community (which, of course, is essentially all publicly visible)
Those metrics tell us everything we need to know about Mozilla's efforts these past few years: people in general are not happy with them, to the post of abandoning Firefox.
As of recently, that trend has finally stopped: Firefox has started gaining (very) small amounts of market share recently. The point where the speed of user attrition slowed down was actually slightly before the Australis launch, but after that, the development sped up considerably.
Thus, it looks like Australis helped substantially with winning back users.
Plus, a few years ago, the competition was essentially IE6/7. Not hard to gain market share against that. Now there are several great browsers to choose from. Which I'd say is a fantastic development, even though I work for Mozilla.
Finally, there are the billions of dollars that e.g. Google pours into advertising for Chrome, which might have something to do how market share developed.
PDF is pretty good and people generally like it and don't think twice that they're not using, say, adobe reader.
As for Australis, there were loud outcries when it happened (as it always does with big UI changes, see 3.x to 4) but it quieted down quite fast.
That's because "loud" doesn't equal majority and this vocal minority got used to, moved elsewhere or simply used some addon to keep things how they liked them.
I usually configure PDFs to be saved, so that I can enjoy a native PDF viewer instead.