There are specific types of plugins according to the browsers you are targeting. Internet Explorer uses ActiveX plugins, while Chrome, Firefox and others use NPAPI plugins.
Unfortunately you can't implement both types of plugin in the same Delphi project. You need one project for each implementation.
There's FireBreath, a C++ plugin framework you can use to build poweful browser plugins targetting all major browser platforms with the same code base. You could easily implement a function to check if your app is installed and deploy a FireBreah plugin compatible with IE, Chrome, Firefox, etc.
See: firebreath.org/display/documentation/Getting+Started
If you can't use C++, here are some useful Delphi examples on NPAPI and ActiveX plugins:
NPAPI: https://www.mozdev.org/bugs/attachment.cgi?id=2541
ActiveX: http://www.delphisources.ru/pages/faq/master-delphi-7/content/LiB0126.html
NPAPI plugins are relatively simple to implement and register. For ActiveX you need a little bit more effort, but it's perfecly possible.