If you want to see how a Safari extension is working, or even change its functionality, it’s quite easy.
First, locate the extension. It should be living in your library folder. Note this is a hidden folder so in the finder use Command-Shift-G to put in this location.
Here you’ll see the Safari extensions you have installed. They have a .safariext file extension, but this is really just a xar file.
To extract the extension’s files, use this command
xar -xf /path/to/extension.safariextz
The extension will be extracted into a folder, with the source and resources contained within.
If you want to change something and then package it back up for reinstallation, you can xar the folder back into a single file.
xar -c -f output.safariextz extensionFolder