add firefox extension template

externs/webextension_polyfill/downloads/DownloadItem.hx

@@ -0,0 +1,94 @@
+package webextension_polyfill.downloads;
+
+typedef DownloadItem = {
+	/**
+		An identifier that is persistent across browser sessions.
+	**/
+	var id : Float;
+	/**
+		Absolute URL.
+	**/
+	var url : String;
+	/**
+		Optional.
+	**/
+	@:optional
+	var referrer : String;
+	/**
+		Absolute local path.
+	**/
+	var filename : String;
+	/**
+		False if this download is recorded in the history, true if it is not recorded.
+	**/
+	var incognito : Bool;
+	/**
+		The cookie store ID of the contextual identity.
+		Optional.
+	**/
+	@:optional
+	var cookieStoreId : String;
+	/**
+		Indication of whether this download is thought to be safe or known to be suspicious.
+	**/
+	var danger : DangerType;
+	/**
+		The file's MIME type.
+		Optional.
+	**/
+	@:optional
+	var mime : String;
+	/**
+		Number of milliseconds between the unix epoch and when this download began.
+	**/
+	var startTime : String;
+	/**
+		Number of milliseconds between the unix epoch and when this download ended.
+		Optional.
+	**/
+	@:optional
+	var endTime : String;
+	/**
+		Optional.
+	**/
+	@:optional
+	var estimatedEndTime : String;
+	/**
+		Indicates whether the download is progressing, interrupted, or complete.
+	**/
+	var state : State;
+	/**
+		True if the download has stopped reading data from the host, but kept the connection open.
+	**/
+	var paused : Bool;
+	var canResume : Bool;
+	/**
+		Number indicating why a download was interrupted.
+		Optional.
+	**/
+	@:optional
+	var error : InterruptReason;
+	/**
+		Number of bytes received so far from the host, without considering file compression.
+	**/
+	var bytesReceived : Float;
+	/**
+		Number of bytes in the whole file, without considering file compression, or -1 if unknown.
+	**/
+	var totalBytes : Float;
+	/**
+		Number of bytes in the whole file post-decompression, or -1 if unknown.
+	**/
+	var fileSize : Float;
+	var exists : Bool;
+	/**
+		Optional.
+	**/
+	@:optional
+	var byExtensionId : String;
+	/**
+		Optional.
+	**/
+	@:optional
+	var byExtensionName : String;
+};