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.

Any ideas?