From MozillaZine Knowledge Base
When local HTML files (e.g., those located on your hard drive) are loaded into the browser, the scripts and links within have restrictions on what they can see and do. Those restrictions are determined by the same-origin policy for local files, which this preference controlled. This preference has since been replaced with security.fileuri.strict_origin_policy.
Possible values and their effects
Local documents only have access to themselves.
Local documents have access to themselves and any other documents in the same directory.
Local documents have access to themselves, any documents in the same directory, and any documents in subdirectories. (Default)
Local documents have access to all other local documents, but not directory listings.
Local documents have access to all other local documents, including directory listings.
If you do development with local files and need to be able to retrieve other local files, you’ll need to set this preference to 3. Otherwise, lower settings offer the most security.
First checked in
Has an effect in