It's a cool thing that Firefox added its own translator as add-on. I hope it improves a lot and can become a competitor with Google Translate and Yandex Translate in the future.
Anyway, I tested it a bit on and I must say that for now it still needs to improve translations in some websites.

E.g.

- I tested it on Bandcamp and translated from English to Italian. Below the profile of an artist, there was written "Records" and Firefox Translations translated it to "i record", which isn't correct because there was the name of a label before.
(But at least this error was highlighted with red because I enabled the "Highlight potential errors in red" function, which is very cool in my opinion).

- I tested it on the Firefox Translations repo on Github and translated again from English to Italian. The "About" word of the About section on the top right of the rectangle with files got translated with "A proposito di"... Only a single word like "Dettagli" (details) or "Descrizione" (description) would have been correct.
Still, in the About section, "a webextension" got translated to "una web estensione", while at the beginning of the README.md the result of the translation was "un WebExtension"; both aren't grammatically correct. "un'estensione web" would be correct in Italian.

These are just a few examples to give the idea of what the translation errors can be and I hope this add-on will get better and better.