I've been looking for this
Like in opera there is an extension which allows two pages open at the same time.
i found two others which didnt work - TwoView & Splitscreen
Web Panel Enhancer works perfectly.
it basically opens pages/links/source in the sidebar
install - http://downloads.mozdev.org/editcss/wpe0.1.6.xpi
more info here http://editcss.mozdev.org/indexwpe.html
there is also another extension you could add on which "Expands sidebars outward to the left of the browser window." https://addons.mozilla.org/extension...nfo.php?id=328 havent tried it yet