First off, is the form that you are closing the form that contains the
Main().  In other words is it the entry point for the application.  I know
you said the behavior wasn't happening before adding the Web Browser
control, however, I just want to make sure.

The only thing that pops into my head is that there is some exception
occuring.  Does the process actually end (use Task Manager to find out)

As far as setting the object reference to null, that won't kick off the
garbage collection.  You will want to try calling Dispose on the browser
control first.  However, I don't think this will help.  Have you tried
making a small sample application with the same structure.  What if you
don't use the user control and just put the web browser control on the form?

Post more info and I will try to help.

Best of luck,