Pencil either runs as a Firefox extension or as a standalone application with XULRunner 1.9.2.
Trying to install XULRunner:
rich@rich-laptop:~/apps/pencil$ sudo apt-get install xulrunner-1.9.2 [sudo] password for rich: Reading package lists... Done Building dependency tree Reading state information... Done Package xulrunner-1.9.2 is not available, but is referred to by another package. This may mean that the package is missing, has been obsoleted, or is only available from another source E: Package 'xulrunner-1.9.2' has no installation candidate
So I tried to install it using Firefox by clicking on the install button on the Pencil home page, and the .xpi file was downloaded and opened by an archive manager.