In the Firefox URL box enter:
If the mozilla-plugin-vlc is installed then there should be a "VLC Multimedia Plugin" section. In that section all of the file/MIME types (e.g. MPEG, WAV, WMV etc.) that will be played using the Mozilla plugin will be displayed.
If not all of the MIME types you want to be played using the Mozilla plugin are there then reinstall VLC again, and ensure that in the "Choose Components" section that in the "File type associations" section the desired file/MIME types are selected to be associated with VLC (also ensure the "Mozilla plugin" tickbox is ticked too
).
Cheers, Arite.