Sometimes Firefox does not know hot to handle certain protocols, such as ed2k. This is the shortest way to have it recognize them.

  • type about:config into the Location Bar (address bar) and press Enter;
  • right-click -> New -> Boolean -> Name: -> Value -> false (Replace foo with the protocol you’re specifying);
  • next time you click a link of protocol-type foo you will be asked which application to open it with.

(this and other techniques are described in the Mozilla Knowledge Base)