Good addon, used by a long time. But with Fx4 it needs work yet. It ignores completely the fact that Fx has a (more limited) support for this same feature: Selecting exactly a url not marked as html link give you duplicated entries in the context menu (Fx's and the extension's)
On the other hand, cheers for the support webpage: it explains you exactly the scope and possibilities of the addon, including a detailed tutorial about how it was written. Fantastic.