Firefox – Send a custom HTTP header to one certain host only

Is there a way (Firefox add-on or about:config setting) to send a custom HTTP header for requests to a certain host?

Example: Send the header Foobar: true if (and only if) I request a resource from the host foo.example.com.

Bonus points if the protocol can be specified, too (e.g. only send the header when requesting over HTTPS).

(see my question Use additional “password” in Referer to hide private site? on IT Security SE on why I’d like to do it)

The question Browser extensions to automatically alter HTTP headers? is similar, but looking for any way how to accomplish this (e.g. with the help of local proxies) and also not necessarily for Firefox
You can create custom headers with the add-on Modify Headers, but it doesn't allow me to restrict the hosts, so it will send the custom headers for all requests
There are some add-ons that allow setting custom values for certain HTTP headers per-host:
- RefControl for Referer
- HeaderControlRevived for User-Agent, Referer and Accept-Language
Related questions:

If you are not restricted with Firefox, you can use Google Chrome's extension Header Hacker.

It provides:

custom request headers (you're able either add new headers or override existing ones)
permanent headers for specified hosts (you're able either add new headers or override existing ones)

So that you can solve your problem with second feature.

I'm using the extension a lot and it works fine for me.