accept_dialog
Accept browser dialog. Provide promptText for prompts.
LABSCO SUMMARY
The console output and the click that caused it come from one place.
get_firefox_output is the part a page-level automation tool cannot give you: MOZ_LOG lines, warnings, crashes and stack traces straight from the browser process, filtered by grep, line count or time. Interaction is UID-based rather than selector-based, so a snapshot has to be retaken after navigation and a stale UID fails loudly instead of clicking the wrong element. Two settings decide how much room the agent has: the preset that chooses its tools, and --connect-existing, which attaches to your real signed-in Firefox and needs Marionette turned on — worth turning off again afterwards, since it changes the browser's fingerprint enough to trip bot detection.
What it is
Mozilla's Firefox automation server, driving the browser over WebDriver BiDi: tabs, DOM snapshots, input, downloads, extensions, screencasts and the browser process itself.
What you get
- Snapshot-then-act interaction: take_snapshot returns stable UIDs, and click, hover, fill, drag and file upload all address elements by UID, with resolve_uid_to_selector turning one back into a CSS selector and failing outright when it has gone stale
- fill_form_by_uid sets several fields in a single call, and accept_dialog and dismiss_dialog handle the modal that follows
- Tab control by index, URL or title, plus history navigation and viewport resizing in pixels
- Capture in three forms: full-page and per-element screenshots as base64 PNG, and screencast_start / screencast_stop recording the viewport to a video file, with several recordings able to run at once
- Download handling: set_download_behavior allows or cancels downloads without the native save dialog, and list_downloads reports status and the saved file path for everything since startup
- The browser process itself: get_firefox_output filters stdout and stderr including MOZ_LOG output, warnings, crashes and stack traces; get_firefox_info reports binary path, environment variables and output file location; restart_firefox relaunches with different prefs, profile or environment
- install_extension takes an .xpi or .zip archive, base64 data or an unpacked directory, and uninstall_extension removes it by ID
- A saveTo parameter on the bulky tools writes the full result to a file and returns the path and byte size, with preview echoing back a few characters inline
- Network, console, debugging and profiler tools available behind --tool-preset developer, and Firefox for Android over adb behind --android-device
Requirements
Node.js 20.19.0 or newer and Firefox 100+ installed locally — it is auto-detected, or named with --firefox-path. START_URL decides the page it opens at. Which tools appear is a launch decision: the default basic preset covers pages, snapshot, input, screenshot, downloads, script, screencast and extensions, --tool-preset developer adds network, console, debugging and profiler, and --tool-preset slim drops evaluate_script. Point it at a dedicated Firefox profile rather than your own, since the agent reaches whatever that profile can reach. MIT or Apache-2.0, at your option.
Setup effort
One command — npx @mozilla/firefox-devtools-mcp@latest
USE CASES
01
Reproducing a bug a user reported
Open the page, snapshot it for UIDs, walk the same steps the report describes, then read the browser's own output for the warning or stack trace the page never showed.
02
Checking a download actually lands
Set downloads to save silently so no native dialog blocks the run, trigger the link, then read back the status and the path the file went to.
03
Recording what the page does for a bug report
Start a screencast, drive the reproduction, then stop it for the saved file path. A still of the whole page or of one element covers the cases where a video is more than anyone needs.
04
Testing an extension build
Install the build straight from an archive or an unpacked directory, exercise a page against it, read the browser output for what it logged, and remove it by the ID the install returned.
05
Getting through a long form to the submit button
Fill every field in one call rather than one call per field, attach the file the form wants, submit, and answer the confirmation dialog that follows.
06
Keeping a big page dump out of your context
Snapshots, screenshots and script results take a saveTo path and come back as a file path and byte size, with a short inline excerpt when you ask for one.
31 tools
Counted once with only startup setup. Other versions, modes or optional credentials may expose a different list.
clear_downloads
Clear the tracked downloads buffer.
clear_snapshot
Clear snapshot cache. Usually not needed.
click_by_uid
Click element by UID. Set dblClick for double-click.
close_page
Close tab by index.
dismiss_dialog
Dismiss browser dialog.
drag_by_uid_to_uid
Drag element to another (HTML5 drag events).
evaluate_script
Execute JS function in page. Prefer UID tools for interactions.
fill_by_uid
Fill text input/textarea by UID.
fill_form_by_uid
Fill multiple form fields at once.
get_firefox_info
Get information about the current Firefox instance configuration, including binary path, environment variables, and output file location.
get_firefox_output
Retrieve Firefox output (stdout/stderr including MOZ_LOG, warnings, crashes, stack traces). Returns recent output from the capture file. Use filters to focus on specific content.
hover_by_uid
Hover over element by UID.
install_extension
Install a Firefox extension using WebDriver BiDi webExtension.install command. Supports installing from archive (.xpi/.zip), base64-encoded data, or unpacked directory.
list_downloads
List downloads tracked since startup, including status and saved file path.
list_pages
List open tabs (index, title, URL). Selected tab is marked.
navigate_history
Navigate history back/forward. UIDs become stale.
navigate_page
Navigate selected tab to URL.
new_page
Open new tab at URL. Returns tab index.
resolve_uid_to_selector
Resolve UID to CSS selector. Fails if stale.
restart_firefox
Restart Firefox with different configuration. Allows changing binary path, environment variables, and other options. All current tabs will be closed.
screencast_start
Start recording a screencast (video) of the current page viewport, saving the output to a file in the downloads directory. Returns a screencast id to pass to screencast_stop. Multiple recordings can run at once.
screencast_stop
Stop an in-progress screencast recording started with screencast_start and finalize the video file. Returns the path to the saved file.
screenshot_by_uid
Capture element screenshot by UID as base64 PNG.
screenshot_page
Capture page screenshot as base64 PNG.
select_page
Select active tab by index, URL, or title. Index takes precedence.
set_download_behavior
Control how downloads are handled: allow (save silently to the default download directory), deny (cancel), or reset to default. Avoids the native save-file dialog. Requires a recent Firefox.
set_viewport_size
Set viewport dimensions in pixels.
take_snapshot
Capture DOM snapshot with stable UIDs. Retake after navigation.
uninstall_extension
Uninstall a Firefox extension using WebDriver BiDi webExtension.uninstall command. Requires the extension ID returned by install_extension or obtained from list_extensions.
upload_file_by_uid
Upload file to file input by UID.
BEFORE YOU INSTALL
What to know first
Tools you get
31 tools when we connected.
Requires
- npx on PATH
- Node.js ≥ 20.19.0
- Firefox 100+ installed (auto‑detected, or pass --firefox-path)
Where it runs
Both documented clients run the server on your machine.
CONTEXT COST
Light Standing cost
Rated by the size of the 31 tool definitions that ride along on every turn, not by how many there are
INSTALLATION
Connect your client
Server setup
TERMINAL
npx @mozilla/firefox-devtools-mcp@latestMCP Clients
Claude Code
CLAUDE CODE · JSON
{
"mcpServers": {
"firefox-devtools": {
"command": "npx",
"args": [
"-y",
"@mozilla/firefox-devtools-mcp@latest",
"--headless",
"--viewport",
"1280x720"
],
"env": {
"START_URL": "about:blank"
}
}
}
}Codex
CODEX · TERMINAL
codex mcp add firefox-devtools -- npx @mozilla/firefox-devtools-mcp@latest
