CreateObjectURL blob url is not protected in Firefox

Question

I am trying to save the downloaded file from a secure url (https) in a browser, but I have problems with Firefox.

I use indexedDB to store the file in browser memory, and when the download is finished, I try to save the file on my computer (I use a Mac, but I think it doesn't matter)

I have this piece of code:

var fileRequest = fileHandle.getFile(); //from indexedDB
fileRequest.onsuccess = function(event){

{...}
var file = event.target.result;
var url = window.URL.createObjectURL(file, {type : fileMimeType, autoRevoke : true});

//I did this with form and not with a href because:
//https://bugzilla.mozilla.org/show_bug.cgi?id=979227

var form = document.createElement('form');
                        form.action = url;
                        document.body.appendChild(form);
                        form.submit();

The browser asks to save this file in the "Download" folder, and everything works fine, but always asks for this message:

"The information entered on this page will be sent via an insecure connection and can be read by third parties. Are you sure you want to send this information?"

"", , - , -, .

URL- createObjectURL - URL-, : : HTTPS//blahblah

Chrome ( ).

: (