There are bugs in revisions after 378 - use of ObserveActivity callback causing crashes.
I tried geckofx-9.0 & geckofx-10.0 Code with http observers realization is VERY unstable.
P.S. I seems to me, that adding observers to default realization of WebBrowser is not good by design, just because observers have app wide visibility (For example, if 5 WebBrowser's are created - 5 observers are created and each observer monitors ALL Http traffic of our application. In C# we must minimize of using COM objects, that are not needed for us.