It works fairly well as a basic epub reader. I wish this has more setting options. Thanks.

Does not work. epubs are still redirected to Edge.

Really like this one. EPUB Reader, my old favoriate, had trouble with some epubs as it would cut off the last 4-5 lines of each chapter. This reader seems to work fine. Automatically goes to 2 columns if the window is wide enough, or you can force it to show one or two pages. Has book marks, adjust font size. The only color changes you have are 3 premade schemes: light mode, dark mode, sepia mode. No web mode (where you scroll vertically through a whole chapter), it has page mode only.

negative:
-hyperlink (footnote) not working.
-cut-off table and images and it continues until the next page (image not resized to fit page)
-can't change font and it's size

positive:
-light-dark-sepia theme
-toolbar always open: open book, table of content, bookmark.

what I want from an ebook reader:
-can display all sorts of content: images, table, symbols, footnote, font color. [fail].
-can easily open book with just a click (no need to 'import' to library. well, I guess you can't import book with current firefox anyway). [pass].
-two column paged view. preferably with visible table of content. [pass]
-can change font, font size (relative, not absolute), line spacing, margin etc. [fail].

It's just basic add-on, that can be developed, anyway it looks better now, than many other ones.

I hope there will be possibility to open books online without of downloading (like pdf ones).

Not a bad extion although there is no option to view all your book - it letsyou read only one book that you add at a time. You cannot change the style of size of the font, also, no option to add bookmarks.