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.