Extremely basic.
No page numbers, no settings, cant even scroll the pages using a mouse.

A extensão, me parece, aplica background-color: white por default, e acaba com qualquer box colorido que o e-book possa ter.

Only thing i intensely dislike about this reader is that if you mess up and close the tab, or have a crash, and you don't recall which ePub you're reading, there's no way to go back to it. Surely there should be a way to write a local state file that would remember the previous opened file and position. Setting a bookmark is fine, but doesn't solve this issue.
If that were solved, it would be 5*.
The other issue is that if highlighting and selecting text, if you're heading towards the margin rather than away and you go a tad too far it will can it as a next/previous page activation, even though there is no click. Easy to recover from, but annoying.

It's a nice extension that i would recommend. However, when i open a web adress that ends with .epub, the extension will intercept and try to open it (with error, of course).

Is there a way to temporarily disable this interception?

Exactly what I needed. Works as intended.

It works well to open files from a web link (HTTP://...). However, if I try to open local files (local hard drive or local network), I get endless "open with..." dialogs.
I found a dirty fix : when I open a file from the web, the address always starts with "moz-extension://697be157-ddf5-4bbd-bd58-56b42dd47af5/reader.html", so I bookmarked this address, and it launches a "blank" epubreader interface, from there I can click the top gray "open" menu, so I can browse my local computer manually. It would be a lot nicer if I could just double-click files from Windows Explorer.

It would be nice to make use of the speech synthesizer capacities to read the book out loud with speed controls

Doesn't correctly format japanese books. Should be top->bottom right->left, but even with right to left reading direction, it shows as horizontal lines.

Кажется, единственный способ на Windows читать epub с заменой гарнитуры на предпочитаемую и с инвертированными цветами. Но слишком много недостатков: нет листания Page Up / Page Down (только стрелки влево / вправо); листание вперед стрелками происходит корректно, назад же при новом разделе перескакивает на десяток экранов; ширина столбца в миллиметрах, а не процентах экрана или окна; при переходе в полный экран (F11) снизу появляется белая полоса; нет настройки инвертирования цветов изображений. EPUBReader 2.0.13 (7 March 2021), Mozilla Firefox 87.0b6 (64-bit), Microsoft Windows 10.0.19042.844.

I used this add-on everyday a few years back before it changed because of compatibility and became unusable. Was hoping it changed since then but it looks even worst... I'm really missing that 1.6 version... :'(