Finally, I found the original tweak
Type "about:config" in the FF location bar;
Right-click on the main FF window and pick new --> boolean;
Name the preference "network.protocol-handler.expose.file", and set it to true;
Create another boolean called "network.protocol-handler.external.file", and set it to true;
Right-click one last time on the main FF window and pick new --> string;
Name it "network.protocol-handler.app.file", and set it to "konqueror" (or, if you prefer, "dolphin").