ePub is an open e-book standard that many e-books now offer. These e-books have the extension .epub and can’t be normally read by standard readers.
Once installed, either drag and drop the ePub book in Firefox if it is stored locally or alternatively type the local path where it is stored. If viewing online, simply click the link where the ebook is stored.
After a bit of processing, the ePub file will open and can be read.
Besides this, there is an additional option of “ePub-Catalog” which lists the epub books read for quick reference and save a copy if possible. Select that by either clicking Tools > ePub-Catalog or the brown book icon at top right in Firefox.
Pretty useful add-on overall.