@sindarina @bert_hubert you can select a piece of text, right click and click "Translate with Firefox Translations" to translate only that bit. Additionally the translation panel has settings which allow you to turn it off globally, or on a per-language and per-page basis.
@gabrielesvelto @sindarina @bert_hubert Also, before it was integrated into Firefox, it was available as an optional extension which translates selected text & also offers a pop-up dialog you can copy/paste to & from.
I'm not sure if it's still supported, but it's still available here: https://addons.mozilla.org/en-US/firefox/addon/firefox-translations/