I tried it quick and a few comments:
It looks like both the web page version and browser extension download from storage.googleapis.com, which is not ideal (obviously much better than the text to be translated being sent). IMO, this should be clarified in the extension description. Are there any plans for Mozilla to host this data? (Well, Amazon I guess to match addons.mozilla.org)
https://mozilla.github.io/translate/
I noticed the web page has Polish but not the extension. Is it not considered sufficiently high quality yet? (Looks like that might be the case from a quick test.)
As with others, I first tried to translate a bit of text not the whole page. Another super helpful feature likely already on a todo list is the ability to see alternative translation options.
Thanks again, this is great and I'm surprised it isn't a larger download. I am curious about any size/quality tradeoffs being made.
Also, playing with the web page a bit English->Spanish I see it does a whole text translation such that adding new sentences affects the translation of earlier ones, sometimes in very odd ways (with one combination of words it translated "Spanish->English" to "español-esplén"). It seems to sometimes produce "Spanish" words that don't seem to be actual words in Spanish as best I can tell and even accoding to its own Spanish->English. It seems like a way to indicate a section to be translated as a unit might be helpful.