As I understand, the author's opinion is that Adobe Reader is more secure than pdf.js. I'm not sure I would trust them with maintaining a secure browser in light of that.
Their monetization model is also questionable (I understand it injects ads / referral links).
Never heard of it, quick scan of the page seems to indicate that it is windows/linux only though? I did a quick look at the download page and didn't see an osx build.
That is overall fine but I like to keep my browsers portable across all of my primary operating systems.
[0]:https://www.palemoon.org