Current version of pencil does not support firefox 4. The bare minimum modification needed is to increase the maximum supported version. There should be many breaking changes between firefox 4 and firefox 3.6 at least when pencil is concerned, though I am not an expert on either one of them.

The build.xml was used to build the XPI-package. I noticed that the build script did not include localization in the XULRunner. This prevented the add-on to be launched.

There are no pre-existing templates on this package probably due to a build configuration anomaly.

On mac (10.6.7 + FF 4.0.1) and using this patch it doesn't save the mockups:

"Error saving file TypeError: this.properties[name] is undefined"

The error is manifested when there's a mockup that doesn't have a name. The mockup can be saved when it has been named. I'll look into assigning a default value to an untitled mockup when saving.

There seems to be a host of other problems with this but I've been reluctant to branch out as I don't have the time to manage it properly.

Fixed and forked: http://code.google.com/p/pencil-ff4/