So far, I can't give this add-on any review — I haven't been able to use it yet.
[ I wrote earlier today, 15 Oct. 2014: ]
I have a new installation of Thunderbird version 31.2.0 on a new MacBook Pro. The "Download" button for Woordenboek Nederlands does nothing.
[ Later the same day, I added this: ]
I am still trying to install Nederlandse Woordenboek as an add-on to Thunderbird. I see that the "Download" button I tried to use earlier today has been replaced by an "Add to Thunderbird" button. But pressing it doesn't add it to Thunderbird. Rather it offers the option to save a file called, “woordenboek_nederlands-3.1.1-sm+tb+fx+fn.xpi”, or to open it, and to choose some application to do so. Choosing "Thunderbird" to open the file merely attaches it to a new e-mail message.
This space isn't offered as a forum, but I hope that the developer or some other user will nevertheless suggest a solution. As for the interface to add this add-on, I have to rate it 'nil.'