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You can use google translate or i thought there is a long way to use the firefox-translations add-on but that is more for testers or if you really want to use it but you can except bugs and you need to use nightly.
Even in nightly version I couldn't find the Page Translator.
I didn't test it because there is no Japanese translation.
I recommend the translation extension on Android: TWP - Translate Web Pages: https://addons.mozilla.org/ro/firefox/addon/traduzir-paginas-web
The Firefox-translations add-on is pretty good you can translate websites but also type text in text translation. But the only downside is that you don't have all the languages. Some are completed and some are in beta. It's more western languages then eastern languages. The most important for me is Japanese but is probably one of the difficulties. The difference between google translate and Firefox-translations is that the google version translation is maked by a bot but the firefox-translations is maked by humans. Everything by firefox-translations is translated by humans that's why it has not many languages because they need people who can speak, read, write that language. https://addons.mozilla.org/en-US/firefox/addon/firefox-translations/
Those extensions are available on desktop version of Firefox only. For PC there are several extensions
But I'm talking about Android version of Firefox
translator for the android app
It would be great if we have a Built in translator or a translator addon on the android app
I think it is the only thing that Firefox missing
(Note: similar ideas have been merged into this thread)
Don't just please depend on enough votes or kutos. Essential features suggestion like this should be considered even when the number of kudos or votes are less.
I personally don't like forums (probably every forum follows this) like this that depends solely on the kudos/vote system.