After the books have been pinned and downloaded, disconnecting the internet and then going to https://read.amazon.com gives the same error as when trying to access any other web page, with no internet connection. I have checked the privileges of read.amazon.com, and it is allowed to store offline content. The Firefox version is 16.0 beta, latest update.
It seems that in order to access offline apps such as this one, users need to go to File->Work Offline.
Source:
http://www.eonlinegratis.com/2013/how-can-amazon-kindle-reader-be-accessed-offline-from-firefox/