You will need version 4 or higher of `firefox` https://www.mozilla.org/firefox
to run Pencil as a Firefox Extension. Linux users will need version 4 or higher of either `firefox`, `iceweasel` or `xulrunner` https://developer.mozilla.org/en-US/docs/Mozilla/Projects/XULRunner,
or version 25 or higher of `palemoon` https://www.palemoon.org/.
The Windows installer and OS X archive has everything you need built-in. Windows, Linux, OS X & Firefox Packages are available for download from the Releases Page https://github.com/prikhi/pencil/releases.
You can also install the Firefox Add-on from the Mozilla Add-on Repository https://addons.mozilla.org/en-US/firefox/addon/pencil-prototyping/.
To install the OS X package, unzip the archive and copy the `Pencil.app` folder t