How do I instruct Firefox to allow me to open file:/// URLs on a localhost-server http:// webpage?

Shamelessly stealing from my answer on StackOverflow:

Links to local files on pages that were retrieved via HTTP(S) are deliberately disabled in Mozilla/Firefox, because they can be a security risk, and have been used in attacks in the past.

You can override this behaviour, however.

Short version:

You need to configure Firefox' Security Policy to allow links to local files. This can be done either using the configuration file policies.json (all platforms), or by using Windows Group Policies (only on MS Window).

In both cases, the setting to change is LocalFileLinks - it contains a list of domains where links to local files are allowed.

Example policies.json:

{
  "policies": {
    "LocalFileLinks": ["http://example.org/",
                       "http://example.edu/"]
  }
}

For details, see this article in MozillaZine, and How to access the local filesystem from a webpage in Firefox on Mozilla Support.