Set Firefox to always download PDF and XML files (e.g. for Selenium)

handlers.json

{
	"defaultHandlersVersion": {},
	"mimeTypes": {
		"application/pdf": {
			"action": 0,
			"extensions": [
				"pdf"
			]
		}
	}
}

user.js

//
user_pref("browser.download.folderList", 2); // 2 - save to browser.download.dir
user_pref("browser.download.dir", "/path/to/download/dir");

user_pref("browser.helperApps.neverAsk.saveToDisk", "text/xml");
user_pref("browser.helperApps.alwaysAsk.force", false);

user_pref("browser.download.manager.showWhenStarting", false);
user_pref("browser.download.manager.alertOnEXEOpen", false);
user_pref("browser.download.manager.focusWhenStarting", false);
user_pref("browser.download.manager.useWindow", false);
user_pref("browser.download.manager.showAlertOnComplete", false);
user_pref("browser.download.manager.closeWhenDone", false);

user_pref("browser.download.viewableInternally.previousHandler.alwaysAskBeforeHandling.xml", false);
user_pref("browser.download.viewableInternally.previousHandler.preferredAction.xml", 0); // 0 - saveToDisk
user_pref("browser.download.viewableInternally.typeWasRegistered.xml", true);

// default is 0, which would overwrite pdf in handlers.json
// https://searchfox.org/mozilla-central/rev/aec7c53cdbbff65305d41c9d805a70efc0e902ed/toolkit/components/pdfjs/content/PdfJs.jsm#142-145
user_pref("pdfjs.migrationVersion", 2);