Got Access Denied for Downloading

I’m writing a web-extension on Firefox Nightly to automatically download some files in the background using downloads API[1].
It works well except when Firefox is launched by WebPagetest, complaining it doesn’t have write permission to user’s Downloads folder even thought I’ve set the full access to Everyone.

Does anyone know if WebPagetest is doing any protection like this? I didn’t find any code doing this, just want to make sure I’m right, thanks.


Update: while the browser is still loading a page, I try to open a new tab and download a file, still get the access denied error when I click “save file”.