One of the questions I get asked a lot is how to set default application handlers in Firefox (the Applications page in preferences). Most companies have resorted to creating a default mimeTypes.rdf file and loading this into their default profile. I finally took the time to understand how these handlers work and can give you some code to use.

Components.utils.import("resource://gre/modules/Services.jsm"); Services.obs.addObserver(function observer(subject, topic, data) { var handlerSvc = Components.classes["@mozilla.org/uriloader/handler-service;1"] .getService(Components.interfaces.nsIHandlerService); var mimeService = Components.classes["@mozilla.org/mime;1"] .getService(Components.interfaces.nsIMIMEService); // Change "image/tiff" the mime type you want to set the preference for var realMIMEInfo = mimeService.getFromTypeAndExtension("image/tiff", ""); var file = Components.classes["@mozilla.org/file/local;1"].createInstance(Components.interfaces.nsIFile); // This should be the path to the .app file on Mac or the EXE on Windows file.initWithPath("/Applications/Preview.app"); var localHandlerApp = Components.classes["@mozilla.org/uriloader/local-handler-app;1"].createInstance(Components.interfaces.nsILocalHandlerApp); localHandlerApp.executable = file; // The name that will be shown in preferences. // Not used on Mac localHandlerApp.name = "Preview"; realMIMEInfo.preferredApplicationHandler = localHandlerApp; // This says to always use the helper app realMIMEInfo.preferredAction = 2; // useHelperApp // This says not to ask realMIMEInfo.alwaysAskBeforeHandling = false; handlerSvc.store(realMIMEInfo); Services.obs.removeObserver(observer, topic); }, "final-ui-startup");

You can add this code to your autoconfig file.