Was early 2011 that different a place? Anecdotally, I've run PDF.js on a PowerPC desktop from 2005 with a late 2011 version of Firefox, and it was a bit pokier than I'd like, but it was usable for me.

(BTW: Apologies, it looks like I somehow fat-fingered a downvote on your post. Unfortunately there's no way to undo that on Hacker News. No slight intended.)