Like there are many Applications which are just basic but you can have install add-ins for it which extends its functionality in that Application. For example:
Fire Bug in Mozilla Firefox.
How do they design such applications and how does the application accept the module and how can it automatically integrate.
Secondly I do not know if the above process is generic or some language or tool dependent. Can we make such application in WPF or Winforms?
Check out MEF.
The Managed Extensibility Framework (MEF) is a new library in .NET that enables greater reuse of applications and components. Using MEF, .NET applications can make the shift from being statically compiled to dynamically composed. If you are building extensible applications, extensible frameworks and application extensions, then MEF is for you.