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.