The extension has a bug: it doesn’t consider non-ASCII letters as part of a word, so e.g. the «conne» in «déconne» is replaced.

Edit: with the upcoming change in Firefox[1], it should only be necessary to use the `u` flag[2] for `\b` to be Unicode-aware.

[1]: https://bugzilla.mozilla.org/show_bug.cgi?id=1634135
[2]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions#Advanced_searching_with_flags_2