I've been looking forward to the day I could try the results of "project Bergamot" in Firefox. The fact that translations are made client-side is great for privacy-minded users (and should be put forward as an USP when marketing this add-on!).

Regarding this add-on: my first impression is that translations (tried German -> English and Portuguese -> English) are of a high quality. Possible errors are highlighted in red: good idea.

I think the UX still needs some polishing, hence the 4 stars. Why just copy-paste Chrome's translation bar? I wouldn't mind a floating, less vertical space consuming toolbar (like we have in Firefox's Reader View). Also: some options are still missing (translate only a fragment, switch back to original text, no manual found, no preferences found ...). A progress bar would also be clearer than the number of elements remaining to be translated.

But very promising for a first release, and I hope more languages will be added soon! I'd also like to contribute to (improving) Dutch translations.