Unfortunately, it doesn't play well with Firefox multi-account containers. Even if the add-on recognizes my account when I left click on its icon, it does not when I double click on a word and therefore I always get the "Please register to continue" message.

EDIT: I don't think I'm the first one to experience this. The same bug was reported a year ago by another user https://addons.mozilla.org/en-US/firefox/addon/reverso-context-beta/reviews/1237464/