Add-On extension XPI file will download but will not automatically install

Dear Community,

I am looking for some help on letting users installing my add-on.
I have develop a very easy add-on (is a self-distributed) and I am trying to create a webpage link to install the add-on directly from the webpage using the Firefox browser.

I have followed this official instructions:

Every time the user click on the link it will download the XPI add-on file but will not install it automatically or will not provide the typical installation request.
How can I achieve the desired result? I have been trying different solution for a while but I am not going anywhere.

If I create a local webpage on my computer and link the URL to the local XPI add-on file it will actually install it as desired (showing a pop-up authorization request).

Thank you in advance for any help!

Edit: my add-on is a self distributed one.

I think your server needs to serve it with proper header:
Content-Disposition: inline

More info:

Or maybe just not serve it with attachment value is enough, this is an example response that works as expected: