We have included a media filter in Firestorm; this will let you control what media stream you connect to, allowing you to whitelist or blacklist any media stream.
If enabled when a stream starts to try playing you will get a window on your screen that lets you choose what you want to do.
Your choices are:
You can access the media list through Preferences → Sound & Media → Manage Media Sites.
The media List allows you to add or remove URLs from either the whitelist or the blacklist. Note that you can use the Add button and manually add a URL.
See this page for documentation on Firestorm 4.6.9 (42969) and earlier.