Was having little luck with printing-to-PDF and addons until I saw a comment to the effect that Firefox has a command line interface since many version ago. Just press shift-F2 and a prompt appears at the foot of the Firefox window. Entering ‘help’ lists the available commands, one of which is ‘screenshot’.
You can get help for screenshot with ‘help screenshot’ but the option format seems to be out of date (in version 22.0). The way to enter the command line swithces is to precede them with ‘–‘. As soon as you type the double-dash you should be prompted with the available arguments. So for example to copy the entire current webpage to your clipboard, you’d type shift-F2 (the CLI doesn’t seem to notice if you change tabs) and enter:
screenshot --clipboard --fullpage
To save the whole current webpage press shift-F2 and enter:
screenshot screen.png --fullpage
Hope this helps!