WM_COPYDATA is a message that must be handled at the receiving end. You cannot just send data and hope that the browser understands what you want.
WM_SETTEXT may work on MSIE but not on Mozilla.
Well, in general, a we browser is expecting and (hopefully!) excepting data only from the web server it is getting it's web contents from, you can't just push data from a locally running application to it.
The closest possibility for that would be a plugin/add-on specifically written for a certain web browser and that is nothing that you can (AFAIK) with Pelle's C...
Ralf