The Reader "replacement" PDF viewer in Firefox is limited and buggy compared to the real thing. We also found it literally unusable for our purposes when it launched, and we routinely disable it on new installations where we still use Firefox at all (which is basically only test machines for web developers now anyway, because of exactly the recent attitude from Mozilla illustrated by this discussion).
I have avoided Reader for years by using Safari, and later Chrome, which each have their own built in PDF renderers. I'm glad that Firefox has caught up, it's one of the things that has kept me from using Firefox. I read a lot of PDFs, and loading the big, slow, clunky Reader plugin, or downloading the PDF, is a non-starter.