How to open "SaveAs" dialog box in Firefox browser?

execCommand("saveas") is not supported in Firefox.

It is generally a bad idea to try and include direct local access as part of your web app program design. There are a lot of restrictions on doing local access, and for good reason. The execCommand() function only works in IE, and only if the user's security settings are low enough.

If you have access to the server side logic you can send a request for the file to the server, and then have the server set the HTTP Response MIME type to a made up value like "application/unknown" that will get the browser to open a dialog box to save the file.