Forget dodgy addons that locked up my browser for about a minute per screenshot taken. Turns out Firefox has the capacity build in. Type shift-F2 to open the terminal and then use #BEGINSRC nil screenshot ~/tmp/page.png #ENDSRC or

screenshot --fullpage ~/tmp/page.png

Really useful to have learnt about this when grabbing stat blocks from the Pathfinder SRD for session notes.