Firefox extensions contain the source code anyways, find the "xpi" file, which is actually a zip file, unzip it, and explore.
Not to discourage asking for help, just that the source not being on github isn't super relevant.
Not to discourage asking for help, just that the source not being on github isn't super relevant.
Looking around on the ScrapBook website[0] reveals that "gomita", the author of the plugin, has a GitHub profile[1]. Sadly without a repository of ScrapBook source code. Still, you could try contacting gomita and ask if they want to put it there to help with reverse engineering it? Or maybe even document how it works.
[0] http://www.xuldev.org/scrapbook/
[1] https://github.com/gomita/