Customizable, and doesn't require more than one permission.

Two clicks to open a webpage via "translate.google.com/" in a new tab. (Not perfect, but the simplest I've found)
You can set a default language and a fall-back language, and hide/show the context menu entry.

Кроме того, что не работает на addons.mozilla.org и некоторых других сайтах (скорей всего это проблема гугл переводчика), больше особых претензий нет.
Может переводить выделенный текст, всю страницу и так, как мне нужно. Есть все необходимые настройки.

Need one more feature: input language.

Auto detection doesn't always work well, especially when translating a small passage of text. Japanese is often recognized as Chinese, and you'd never know it, because it display say what language the selected text is detected as. Similarly, Dutch could be detected as German, Spanish as Portuguese, etc.

Input language should perhaps be a priority list of languages you most likely want to translate, and have the translation engine prefer those over whatever is autodetected, if possible.

I've tried it and actually it works great!!
Besides, it needs some improvements, let's say:
+ Language of selected text: Japanese
+ Target language: English

Currently, when I select Japanese text, it translates text to English. It's OK in this case.
But when I select English text (that is same as Target Language), and my setting for second language is FRENCH, then I have to click on the icon button on the toolbar to see the translation to FRENCH, and cannot see directly on Translation Panel.

==> IMPROVEMENT: it is expected it AUTOMATICALLY translates to SECOND LANGUAGE and directly displays on Translation Panel in case Target Language is same as language of selected text.
Many thanks.

Almost perfect, but has no audio function and no reverse translation.

It would be nice to have the second language option for translation button.

I like the simplicity, though.

Really good! Very responsive and nice looking. Waiting for possibility to change font color.

Good addon, I have not to open new tabs / new windows (to translate.google.com). Just a popup

TL;DR: Works as intended, even on Firefox Quantum 57.0.4 (64-bit).

As the name implies; it's simple, and it translates, while a great majority of other translation add-ons DON'T.
More often than not they seem only capable of redirecting you to a Google Translate page containing whatever website or blog or piece of text you wanted to translate. Seriously, anyone with a computer and two hands could do that. And that's why this one stands out: it might not be spectacular or anything, but at least it's capable of translating whatever you highlight and does so in a neat little textbox, all without disrupting your workflow.

Even though the actual translations can be a bit rough from time to time, i'm usually able to make sense of it nonetheless.
So if you're looking for a simple and easy to use add-on for on-the-fly translation, this might be your best option.

-Simplicity in its best.
-Maybe you should consider a setting for translating only by selection.
-Sometimes translate button doesn't appear.

Excellent add-on. Please add a translation by clicking the hotkey. Thank you!

This is a great and well designed translation add-on!

However:
- Automatic switching to second language seems to be not working when selecting text. (reported bug)
- Switching language in the popup box (after clicking the icon) would be good to remember at least till the end of browser session (or another change).
- Using the language (selected in the popup box) as "first" for the context menu action (at least till the end of browser session) would be a great feature.

And again: this is a great and well designed translation add-on!
(I will give 5 stars after fixing the above issues)

He does his job well

翻訳システムにDeepLを追加して頂けると更にうれしいです。

good，this is what I wanted！！

Can I recommend two new customization options?

1.- Add "translate selected text to google translate" for advanced translate options (as listening to the translation)
2.- Put "translate selected text" in the top of the list (or even better, select the options order)

So far is a great addon, but I think It need some little options to be better

Thanks :)