Browser launch flags for Chrome, Firefox, and Edge

Control browser launch behavior for debugging, automation, and runtime experiments.

Looking for chrome://flags or edge://flags? Type it into your address bar, which is the browser’s built-in page. Firefox has no firefox://flags; use about:config instead.If you’re toggling flags to test or build a browser extension, Extension.js applies them per-project automatically across Chrome, Edge, and Firefox, so you don’t pass flags by hand on every run. Create your first extension in 30 seconds →

Tune browser launch behavior without changing extension source code. Extension.js merges browser flags from your extension.config.* and applies them in dev, preview, and start flows.

Does Firefox have `firefox://flags`?

Firefox does not use firefox://flags the way Chromium browsers use chrome://flags or edge://flags. Firefox exposes runtime toggles through about:config (preferences) and accepts launch flags from the command line. If you searched for firefox://flags, firefox //flags, browser://flags, mozilla://flags, or about flags firefox, you are probably trying to change browser behavior during extension development. In Extension.js, do that in two places:

Use browserFlags for launch-time flags Extension.js passes to the browser binary.
Use Firefox preferences for repeatable Gecko runtime behavior that would otherwise live in about:config.

Both work for Chrome, Edge, and Firefox extension development from the same Extension.js project.

Flags in other browsers

Extension.js launches and configures any Chromium-based browser the same way.

brave://flags, opera://flags, vivaldi://flags, yandex://flags: all Chromium-based, so they behave identically to chrome://flags, and Extension.js manages them per-project.
internet://flags and browser://flags: these are not real browser schemes. You are most likely looking for chrome://flags (Chromium) or about:config (Firefox).
edge://flags: Edge is Chromium, so it works exactly like Chrome in Extension.js.

Building an extension that needs specific flags at launch? Set them once in extension.config.* and Extension.js applies them every run. See browserFlags below.

Template examples

`new-browser-flags`

See browser flags in action with a new-tab extension that configures launch behavior.

npx extension@latest create my-extension --template=new-browser-flags

pnpx extension@latest create my-extension --template=new-browser-flags

yarn dlx extension@latest create my-extension --template=new-browser-flags

bunx extension@latest create my-extension --template=new-browser-flags

deno run -A npm:extension@latest create my-extension --template=new-browser-flags

Repository: extension-js/examples/new-browser-flags

How it works

Configure flags in extension.config.*:

browser.<target>.browserFlags
commands.dev|start|preview.browserFlags
Optional excludeBrowserFlags to remove defaults or user flags (behavior depends on whether you target Chromium or Firefox).

Override order: browser defaults → command defaults → CLI-selected command context.

Flag capabilities

Config key	What it does
`browser.<target>.browserFlags`	Sets default launch flags for a specific browser target.
`commands.dev.browserFlags`	Adds or overrides flags for `dev` runs.
`commands.start.browserFlags`	Adds or overrides flags for `start` runs.
`commands.preview.browserFlags`	Adds or overrides flags for `preview` runs.
`browser.<target>.excludeBrowserFlags`	Removes matching default or user flags for a target.
`commands.<name>.excludeBrowserFlags`	Removes flags in a command-specific context.

Example configuration

export default {
  browser: {
    chrome: {
      browserFlags: ["--disable-web-security", "--auto-open-devtools-for-tabs"],
      excludeBrowserFlags: ["--mute-audio"],
    },
    firefox: {
      browserFlags: ["--devtools", "--new-instance"],
      excludeBrowserFlags: ["--devtools"],
    },
  },
};

Chromium vs Firefox behavior

Chromium family (chrome, edge, chromium, chromium-based)
- Starts from an internal default flag set, then appends your browserFlags.
- excludeBrowserFlags removes matching default flags (exact-match filtering).
- Extension.js manages --load-extension=... and filters it out from user-provided flags.
Firefox/Gecko family (firefox, gecko-based / firefox-based)
- Uses user-provided browserFlags (no large default flag bundle like Chromium).
- excludeBrowserFlags removes user flags by prefix matching.

Default Chromium flags

Extension.js applies these flags automatically when launching Chromium-family browsers. Use excludeBrowserFlags to remove any you do not need.

Flag	Purpose
`--no-first-run`	Disable first run experience
`--disable-client-side-phishing-detection`	Disable phishing detection
`--disable-sync`	Disable sync to avoid account prompts
`--disable-component-extensions-with-background-pages`	Disable built-in extensions not affected by `--disable-extensions`
`--disable-default-apps`	Disable installation of default apps
`--disable-features=InterestFeedContentSuggestions`	Disable Discover feed on new tab page (NTP)
`--disable-features=Translate`	Disable Chrome translation
`--hide-scrollbars`	Hide scrollbars from screenshots
`--mute-audio`	Mute any audio
`--no-default-browser-check`	Disable default browser check prompt
`--ash-no-nudges`	Avoid user education nudges
`--disable-search-engine-choice-screen`	Disable search engine choice screen
`--disable-features=MediaRoute`	Disable Chrome Media Router background networking
`--use-mock-keychain`	Use mock keychain on Mac to prevent blocking dialogs
`--disable-background-networking`	Disable background network services
`--disable-breakpad`	Disable crashdump collection
`--disable-component-update`	Disable component updates
`--disable-domain-reliability`	Disable domain reliability monitoring
`--no-pings`	Disable hyperlink auditing pings
`--enable-features=SidePanelUpdates`	Ensure side panel is visible
`--disable-features=DisableLoadExtensionCommandLineSwitch`	Allow `--load-extension` at the command line
`--disable-features=ExtensionDisableUnsupportedDeveloper`	Keep unpacked extensions alive across `runtime.reload()` (Chromium 152+)
`--enable-unsafe-extension-debugging`	Allow Chrome DevTools Protocol (CDP) extension management (Chrome 126+)
`--silent-debugger-extension-api`	Suppress the “X is debugging this browser” infobar

Always-on session flags

Beyond the default table, every Chromium launch gets a fixed block of session-stability flags. They keep background timers, occluded windows, and renderer throttling from distorting automated and AI-driven sessions. excludeBrowserFlags cannot remove them:

--disable-background-timer-throttling
--disable-renderer-backgrounding
--disable-backgrounding-occluded-windows
--disable-features=TranslateUI
--disable-hang-monitor
--disable-prompt-on-repost
--memory-pressure-off
--max_old_space_size=4096
--disable-dev-shm-usage

In dev mode only, Extension.js also wires the debugging channel with three flags: --remote-debugging-port=<derived port>, --remote-debugging-address=127.0.0.1, and --remote-debugging-pipe. Production-mode launches (start, preview) skip this trio.

Feature-switch merging

Chromium honors only the last occurrence of a repeated switch. Extension.js therefore merges every --enable-features and --disable-features occurrence into one comma-joined switch each. Defaults, config, and environment values all merge, and duplicate feature names are removed. You can safely pass --disable-features=Foo in browserFlags without erasing the defaults.

Environment switches

EXTENSION_BROWSER_FLAGS appends whitespace-separated flags to every launch. It is applied after config flags and after excludeBrowserFlags filtering, so excludeBrowserFlags cannot remove flags that come from this variable.
EXTENSION_HEADLESS=1 (or true) forces --headless=new, unless a --headless flavor was already passed through config or environment flags. Use it as a focus-steal guard for automated sessions.

In Docker, continuous integration (CI), or containerized environments, Extension.js also applies --no-sandbox and --disable-setuid-sandbox automatically. On Linux it detects a container through any of these signals: CI=true, a /.dockerenv file, a /run/.containerenv file, REMOTE_CONTAINERS=true, CODESPACES=true, or a set container variable.

Supported targets and references

Browser	Usage	More information
Chrome	`extension dev --browser=chrome`	Chrome Flags
Edge	`extension dev --browser=edge`	Edge Flags
Firefox	`extension dev --browser=firefox`	Firefox Flags
Chromium-based	`extension dev --browser=chromium-based`	Chromium Flags
Gecko-based	`extension dev --browser=gecko-based`	Firefox-based browsers share the same flags as Firefox.

Use binary flags when needed for engine-based targets:

--chromium-binary=...
--gecko-binary=...

Best practices

Add only necessary flags: Minimize long flag lists to reduce flaky or non-portable setups.
Prefer excludeBrowserFlags over replacing defaults: Remove only what conflicts with your workflow.
Do not pass --load-extension manually: Extension.js manages extension loading flags internally.
Validate per browser family: A flag working in Chromium may be invalid or ignored in Firefox.

Next steps

Learn more about Browser preferences.
Learn more about Browser profile.
Choose a target in Browsers available.
Wire flags through extension.config.*.
New to extension tooling? Start with the getting started overview.

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Browser launch flags for Chrome, Firefox, and Edge

Does Firefox have `firefox://flags`?

Flags in other browsers

Template examples

`new-browser-flags`

How it works

Flag capabilities

Example configuration

Chromium vs Firefox behavior

Default Chromium flags

Always-on session flags

Feature-switch merging

Environment switches

Supported targets and references

Best practices

Next steps

​Does Firefox have firefox://flags?

​Flags in other browsers

​Template examples

​new-browser-flags

​How it works

​Flag capabilities

​Example configuration

​Chromium vs Firefox behavior

​Default Chromium flags

​Always-on session flags

​Feature-switch merging

​Environment switches

​Supported targets and references

​Best practices

​Next steps

Does Firefox have `firefox://flags`?

Flags in other browsers

Template examples

`new-browser-flags`

How it works

Flag capabilities

Example configuration

Chromium vs Firefox behavior

Default Chromium flags

Always-on session flags

Feature-switch merging

Environment switches

Supported targets and references

Best practices

Next steps