In Preferences->Downloads->View and edit actions the default actions for many types are invalid (eg. AVI->open with Windows Media Player). Ideally the action would be defined by the desktop (ie. KDEs file associates), alternatively mplayerplug-in can be used; at the moment types like AVI aren't associated with it, although mpeg is. Firefox doesn't even report an error when the default action fails, maybe that should be noted upstream.
It is refering to mplayerplug-in for wmv closing invalid
Created attachment 83094 [details] download-actions.png screenshot showing download actions.
I removed the .mozilla directory, restarted firefox, and took this screenshot showing the problem. Installed packages: mozilla-firefox-1.5.0.1-r3 net-www/mplayerplug-in-3.21
As I stated early it is due to mplayerplug-in has nothing to do with firefox. If you doubt this remove mplayerplug-in remove your .mozilla and test and you will see what I am telling you.