Page Translator 3 by Hellboy.

Translates any page using Google Translate or Yandex Translate by opening the appropriate URL.

By default, translation is done via a page action in the address bar. The translate icon will only appear if the page is in a foreign language. Page Translator determines if the page is in a foreign language by comparing the list of languages you have specified in Firefox as your preferred languages (Preferences > Content > Languages) against the page language as determined by a built-in Firefox API.

Firefox for Android does not yet support language detection, so the translate icon will always be visible.

Yandex "Auto-Detect" doesn't work correctly. For text translate, when page open, is needed to press "space" or "enter" to detect the auto.

In the options you can set the extension to redirect to the translation website automatically if a foreign language is detected, or to always show the translation icon.