The popular Translate Web Pages addon[0] was recently updated to silently send all content that is hovered over to Google Translate or Yandex, by default. Needless to say, a huge breach of trust considering how much text you might hover over in your browser.
Previously, you would need to click the addon icon to enable it for specific web pages.
Here is the GitHub issue tracking the issue: https://github.com/FilipePS/Traduzir-paginas-web/issues/109
[0] https://addons.mozilla.org/en-US/firefox/addon/traduzir-paginas-web/