There's a lot going on in web pages. Tools like Firebug and Web Inspector help a lot when trying to understand what's going on within a web page, but if there are plugins like Flash, Windows Media or QuickTime involved things get even more confusing. I've written a little tool called plugin logger that monitors and logs all interaction between the plugin and the browser over the NPAPI interface.
Right now it's only been built and tested on Linux but it shouldn't be too hard to port to Mac or Windows. You can get the latest version off github.