Interesting concept that I would say has definitely helped me with my learning of a 4th language.

It would be better if more appropriate words are chosen for translation though. As a complete beginner who didn't even know the new language's equivalent of I/you/we/they/he/she/it, I was really dumbfounded when Toucan chose relatively uncommon or niche words like "revenue", "president", "concern", "galaxy", and "certification" when there are plenty of other simpler and more commonly used words available. I already had "beginner" level chosen. Perhaps a more refined learning level system is required?

Very intuitive and easy to use. However, I know many friends who could get a lot of use out of this extension if it supported Arabic as a native language, so they could learn English and many others. Please let us know when that is made possible. :)

It's a very interesting idea for an extension and I'm glad it was made. As others have said, it does have performance issues. On a high performance desktop PC it's not terrible but the lag is still very noticeable.

Sometimes the replacements don't make logical sense in the context. Nouns are fine, and it mostly replaces nouns, but sometimes it replaces a verb and for example in German it will put the infinitive form of a reflexive verb in place of an English verb, so in the sentence "I can't afford it" you see "I can't [sich leisten] it", which doesn't make sense. But I suppose it'll never make sense putting directly translated foreign words into an English sentence. I still can hover over it and learn the word, and thus expand my vocabulary.

Sometimes it translates a word with the wrong translation for the sentence because a word exists in English with multiple meainings. For example, "harbour" as a verb is translated into the noun form, which no longer makes sense. Or for another example of a mistranslation, it translates "vitamin A defieniency" into "vitamin [a deficiency]" where the part in square brackets is in another language. This is because it thinks 'A' is the indefinite article, but it's the actual name and shouldn't be translated.

What would be really cool is if it could replace more of the words around the word, or even entire sentences. The way it currenly works can be confusing and it reduces readability to see a random noun in German but the connecting words and articles in English. My brain expects to see the other words in German as well if I'm reading German, and I'm worried I'm training my brain to confuse the languages.

It's especially confusing when a word exists in both languages but with different meanings. For "a disc jockey" it translates "a disc" literally and leaves the word jockey, which no longer makes sense. It should have been translated into a single word. "Box office" should be translated as one word "Kinokasse" but instead it leaves "box" and just translates "office" into "Büro" which is meaningless. A literal office in a box?

I suppose it might be too difficult to translate partial sentences and too difficult for learners to translate entire sentences. The dream would be a machine learning algorithm that knows what I know and replaces what it knows I will understand, so that over time I end up reading mostly German with only the occasional English word.

For now, I think this add-on will be good for improving my vocabulary via exposure to new words and regular practice that I otherwise wouldn't have, but it is not without problems. For me it's still useful enough to have enabled, just for the repeated exposure.

This addon works as it says. One caveat (at the time of writing) is that sometimes it slows down the browser to a standstill. I've been told, that they're working on it though, so one can hope. You can disable it on specific sites, or entirely, if you do not want to deal with it for a while.

I use Toucan to learn Japanese and so far, my experience has been great! Of course, like any other language learning app, there are limits to how much of the language you can learn through it, but as far as learning vocabulary (and in the case of Japanese, the alphabets) goes, I think this is a fantastic concept!
Of course, since Toucan is an ever-developing add-on, there are things I would like to see that would make it an even better experience, such as including some kanji along with the hiragana and katakana in the case of Japanese, or translating some phrases to immerse the user even more in aspects of the language such as syntax (but I realize that is probably much harder to implement).
The dev team is really communicative and it seems to me that they really care about providing the best possible user experience, which is another huge plus. I'll definitely let them know about what I feel is missing in the current version.
Full disclosure, the Toucan team generously supplied me with a month of premium membership to fully try the add-on out, but I believe that everything I said also applies to the free version.