Filtering of browser documents before rendering? (bestly XSLT)

I want to filter a video advertisment before evening news, which is downloaded by a Flash player from this URL. I know how to remove the first <switchItem> from the document. The question is:

How can I access the document between it's download and passing it to the Flash player?

I am ok with 开发者_StackOverflowFirefox-only solution.

No Internet Explorer solution please.

Proxy server solution not preferred, that's an overkill for my case.