Surprisingly it works, which is great! It was basically exactly what I needed. A few issues though: it wasn't obvious to me that the file will be downloaded automatically, I downloaded it a few times until I realized that it had downloaded in the download menu (perhaps this is more of a Firefox problem, but if you could do anything about that, that would be great); also the screen where it says "the assets you download will be listed here for reference", doesn't work, it doesn't show anything; finally, it would be nice if it would be possible to download directly as a pdf, instead of an epub. All in all, great add on though!

Doesn't download the complete book.
When the epub is downloaded, it only contains all book sections; for instance Chapter 1 [sub-topics,..], 2, 3 and so on, but when clicking on each sub-topic, it doesn't display any information. This happens with all books.

Posting URL
https://learning.oreilly.com/library/view/ccnp-and-ccie/9780135262047/part7.xhtml

I see, I guess I am wrong, my bad.
I am using E-book viewer in Ubuntu.
I will try it with another viewer.

it worked as expected for English contents of Oreilly but epub format was corrupted for Korean contents of Oreilly.

Since I cannot reply to your response, I edited my review instead. I tested these two books https://learning.oreilly.com/library/view/kubeonetiseureul-hwalyonghan-keulraudeu/9791162248249/ and https://learning.oreilly.com/library/view/keulraudeu-neitibeu-keulraudeu/9791162243183/ . This extension successfully downloaded epub files for both but the epub contents formats were corrupted with very big and overlapped fonts for both.