Плюсы: есть темная (серая) тема. Минусы: нет выбора гарнитуры, нет выбора цвета фона и цвета текста, нажатие на кнопки увеличения / уменьшения кегля ни на что не влияет, нет поддержки листания кнопками Page Up / Page Down.

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].

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.