---
title: Firefox Local Mode
link: https://firefox-source-docs.mozilla.org/devtools-user/local_mode/index.html
source: lobste-rs
published: 2026-10-21T15:47:59Z
updated: 2026-10-21T15:47:59Z
first_seen: 2026-10-22T01:26:25.644952883Z
authors:
- firefox-source-docs.mozilla.org via lim
labels:
- browsers
summary: Comments
content: extracted
html: 2026-10-21-firefox-local-mode.html
remote_preview:
  url: https://firefox-source-docs.mozilla.org/_static/firefox-wordmark.svg
  alt: Logo
---

## What is the “Local Mode”

This feature allows you to register multiple mappings in order to load local files through custom https:// URLs. This prevents having to spawn a HTTP server to test a web page. This makes it easy to test all Web APIs, including the ones being restricted to [Secure Contexts](https://developer.mozilla.org/en-US/docs/Web/Security/Defenses/Secure_Contexts) (i.e. using https). This also helps having many distinct origins (domain) for each experiment / project and have dedicated storage/cookies for each.

These mappings are only accessible from Firefox (this can’t be used by any other program, or malicious software on your computer/local network).

These mappings are only functional when DevTools are opened.

## How to use the “Local Mode”

This feature shipped in Firefox 153.

You can register mappings from DevTools’ Options panel:

![The Local Mode settings in DevTools options panel](https://firefox-source-docs.mozilla.org/_images/options-panel.png)

From there you can register as many mappings as you need.

Each mapping defines:

- the origin (domain)

  The custom origin to expose via https URL.

- the local folder

  The local folder where to load files from.

On the screenshot, it loads `https://firefox.localhost` from `/home/alex/app`.

## Loading file URL notification

When loading any file:// URL with DevTools opened, you should see the following notification at the top of DevTools:

![The notification shown when loading file:// URL](https://firefox-source-docs.mozilla.org/_images/file-notification.png)

The first action `Add To Settings` will bring you to the Options panel and register a new mapping for the loaded file URL.

The second action `Try it` will register a transient mapping to load the current file or folder from a https URL. The mapping will disappear as soon as you close DevTools or Firefox.

## Manage mappings via preferences

Mappings can also be managed in about:config via preferences, so that you can easily import and transfer them between profiles, or via automation.

Each mapping is defined by three preferences:

- `devtools.local-mode.mappings.${index}.origin`: string for the mapping’s origin. (e.g. “firefox.localhost”)

- `devtools.local-mode.mappings.${index}.path`: string for the absolute path to local folder (e.g. “/home/hello-world”)

- `devtools.local-mode.mappings.${index}.disabled`: optional boolean to disable this mapping and where `${index}` is an incremental integer counter to uniquely identify each mapping.

Here is an example of mappings you can control from about:config:

![List of Local Mapping preferences displayed in about:config](https://firefox-source-docs.mozilla.org/_images/mappings-in-about-config.png)