Applications and download actions list is empty
One possible cause of an empty file types and download actions list (e.g., Firefox 3 "Applications" list) on Windows systems is the presence of the npbittorrent.dll file in the Firefox plugins folder  . If you see the BitTorrent plugin in your about:plugins list, then exit Firefox, open the Firefox installation directory\plugins folder (C:\Program Files\Mozilla Firefox\plugins in a typical Windows install) and either remove the file "npbittorrent.dll" or rename it Xnpbittorrent.dll.
Another possibility is a corrupt mimeTypes.rdf file . Exit Firefox completely and delete or rename the "mimeTypes.rdf" file in the Firefox profile folder and the file will be regenerated (read this for help finding it). This will remove any previously defined download actions. Note that, in some cases, you may need to follow these instructions to completely reset your Firefox download actions and any related preference settings that may exist.
On Linux (Kubuntu), installing the firefox-gnome-support package from the Kubuntu repositories is reported to resolve the issue.