Conkeror is a standalone browser (not to be confused with KDE's Konqueror) I've been using with a similar mission, though I look forward to trying this out and take advantage of FireFox's technologies (debug, pdf.js integration, etc.)
Conkeror is so much Firefox, that you can easily fit the former with most things that work with the latter, including the built-in PDF extension. As a matter of fact, the Conkeror "binary" is just a shell script that launches Firefox or xul-runner with the proper config (something like: firefox -app /usr/share/conkeror/application.ini).
Also worth noting is that Conkeror was the original inspiration for vimperator (and thus for uzbl, dwb, luakit and the likes - all very splendid and worthwhile projects), and that this message is written on an editor spawned by conkeror (which, surprisingly enough, happens to be vim).
