> ## Documentation Index > Fetch the complete documentation index at: https://docs.browspark.krishm.dev/llms.txt > Use this file to discover all available pages before exploring further. # Firefox and Zen support > Share existing Firefox and Zen tabs with the extension, or launch separate developer sessions with WebDriver BiDi. Firefox and Zen have two connection options. The Firefox extension shares existing tabs and signed-in sessions through the same dashboard as Chromium. Developer mode launches a separate profile through WebDriver BiDi. Both keep the same MCP tool names, with different capability limits. ## Install the Firefox extension Use Firefox 153 or later, or a Zen build based on Firefox 153 or later. Build the current source: ```bash theme={null} git clone https://github.com/uncaughterrs/browspark.git cd browspark bun install bun run package ``` This creates `dist/firefox-extension/` and `dist/browspark-firefox-extension.zip`. The separate `dist/browspark-extension.zip` is for Chromium. 1. Open `about:debugging#/runtime/this-firefox` in Firefox or Zen. 2. Choose **Load Temporary Add-on** and select `dist/firefox-extension/manifest.json` or the Firefox ZIP. 3. Register the companion from the same source revision with your MCP client: use command `bun` and the absolute path to `companion/src/index.ts` as its argument. This keeps the extension and companion protocol versions aligned before a release is published. 4. Open the Browspark toolbar icon. If shown, choose **Allow website access**, then **Enable Firefox automation** to grant user-script permission. Share the HTTP or HTTPS tabs the agent may access. 5. Ask the agent to call `browser_status`, then use the listed `tabId` or choose the Firefox profile's `browserId` when opening a new tab. The package is unsigned. Temporary installation lasts until the browser restarts; reload it through `about:debugging` afterwards. Permanent installation in standard Firefox needs Mozilla signing. See [Mozilla's temporary installation instructions](https://extensionworkshop.com/documentation/develop/temporary-installation-in-firefox/). The extension requests access to all websites (``) because Firefox requires this permission for native tab screenshots, plus user-script execution for page automation. Browspark still limits automation to shared HTTP/HTTPS tabs and checks sharing before each command. It connects to your local companion; content returned to your MCP client may be processed by your chosen AI provider. Firefox browser pages, add-on pages, protected Mozilla sites and local files are unavailable. ## Extension tool coverage The extension provides shared-tab automation through Firefox WebExtension APIs and a sandboxed user-script world. It does not expose a native debugger. See the coverage notes below before choosing between an existing shared tab and a separate developer profile. | Area | Firefox and Zen extension behavior | | ---------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | Tabs | List, open, close and activate shared tabs; navigation, reload, back and forward. | | Page content | Snapshots and refs, reading, extraction, waits and DOM inspection/editing. Same-origin frame DOM is accessible; cross-origin frames are not. | | JavaScript | Evaluation in an isolated user-script world, with DOM access but without the page's JavaScript globals or extension APIs. | | Interaction | Ordinary left clicks, simulated keyboard events, filling, selection and scrolling. Hover, drag, non-left clicks, multiple clicks and click modifiers are unavailable. Events are untrusted (`isTrusted: false`); websites that require native input can reject them. | | Screenshots | Capture the selected tab through Firefox's tab-capture API. | | Stop controls | Unshare or stop access in the dashboard. The in-page agent overlay is unavailable. | | Network, cookies and console | Network capture/interception, network-backed policies, browser cookie APIs and console collection are unavailable. Page DOM APIs remain available through evaluation. | | Files and dialogs | Download tracking, PDF printing, native file upload and JavaScript dialog handling are unavailable. | | Developer tools | Raw CDP, Lighthouse, debugger control, early-document scripts, Chrome tracing/profiling and other native DevTools operations are unavailable. | All 43 tool names remain registered. Unsupported actions return explicit errors for the selected Firefox extension tab. Use a separate Firefox or Zen developer session when you need supported BiDi capabilities such as native input or network inspection; that session has its own profile and logins. Use Chromium developer mode for CDP or Lighthouse. ## Launch a separate Firefox or Zen developer browser Install the browser, [register the companion](/connect/agents), and ask your agent to open a Firefox or Zen developer session. For Firefox it calls `browser_session` with: ```json theme={null} { "action": "launch", "browser": "firefox", "context": "firefox", "userRequested": true, "url": "https://example.com" } ``` For Zen, change `browser` to `"zen"` and use its own context, such as `"zen"`. You can run both alongside Chrome and Brave. Use a returned tab ID with `browser_snapshot`, `browser_click`, `browser_read`, or another tool. When opening another tab, specify `mode: "dev"` and the intended `context`. See [using multiple browsers](/reference/multiple-browsers). `browserPath` selects the executable for the chosen browser. Set `BROWSPARK_FIREFOX` for Firefox or `BROWSPARK_ZEN` for Zen in your MCP server's environment; the existing `firefoxPath` option remains supported. Other Firefox-based browsers, such as LibreWolf, can be selected with `browser: "firefox"` and an executable path when their build enables WebDriver BiDi. Custom builds may disable remote automation or omit commands; an executable path does not guarantee the same capabilities in every fork. Profiles persist under `~/.browspark/profiles/.firefox/` for Firefox and `~/.browspark/profiles/.zen/` for Zen (under `BROWSPARK_PROFILES` when set). They are separate from each other and from Chromium profiles, including the `default` context. A context name can belong to only one running browser at a time. Pass the matching `browser` when deleting a stopped context. ## Developer-mode tool coverage Unsupported actions return an explicit error. They do not silently run in Chrome or substitute results from another browser. | Area | Firefox and Zen behavior | | ------------------------ | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | Tabs and navigation | Launch, list, open, close, activate, navigate, reload, back and forward. | | Page content | Snapshots and refs, text/markdown/HTML reading, extraction, waits, and JavaScript evaluation. | | Interaction | Mouse, keyboard, forms, selection, scrolling, file upload, and JavaScript dialogs. | | Output | Viewport, full-page and element screenshots; PDF printing. | | Downloads | Download tracking and saved file paths when the selected build supports BiDi download events. | | Console | Console messages and exceptions collected during a devtools session; evaluation and inspection of live objects returned by `devtools_evaluate` with `returnByValue:false`. Logged objects have searchable snapshot previews; Firefox does not provide live handles for those logged snapshots. | | Elements | Search, inspect and edit HTML/attributes by ref or selector; inline style edits and computed styles. Matched rules, box models, event listeners, forced pseudo states and DevTools overlays are unavailable. | | Accessibility | `devtools_accessibility` with `action: "check"` runs DOM-based checks. Browser accessibility trees and Chrome issue reports are unavailable. | | Network | Request/response logs, headers, status, timing, HAR, cache control, blocking, mocks, resource overrides, and domain policies. Response bodies depend on the browser's BiDi data-collection support. | | Storage | Cookies in the selected context; local/session storage, IndexedDB and Cache Storage through page APIs. | | Sources and workers | Frame/context listing and resource overrides; page-based service-worker list/update/unregister. Script inventories, source maps and worker debugging are unavailable. | | Page metrics | `devtools_performance` with `action: "vitals"` reports the metrics exposed by Firefox's page APIs. | | Raw CDP and Lighthouse | Unavailable. `devtools_cdp` and `devtools_lighthouse` require Chromium. | | Advanced Chrome DevTools | Chrome traces, V8 heap snapshots and CPU profiles, debugger breakpoints/stepping, precise JS/CSS coverage, and Chrome-specific emulation or browser issue reports are unavailable. | Network capture does not include WebSocket/SSE frames, TLS certificate details, remote IPs, or Chrome initiator stacks. Network throttling and service-worker bypass are unavailable. Replaying a request with a body requires its body to be supplied explicitly when the original was not captured. Literal multiline text in `browser_fill` or the `text` argument of `browser_key` is unavailable in Firefox and Zen; it returns an error before typing. Ordinary text and explicit keyboard actions use native browser input. These developer-mode limits apply to both Firefox and Zen. The generated [CDP capability matrix](/reference/capability-matrix) describes Chromium only. `devtools_capabilities` reports the selected tab's backend and capabilities; a supported domain does not imply that every Chrome command in it is available. ## Launch options * `headless: true` runs the browser without a window. Use screenshots to inspect it; the Chrome live screencast is unavailable. * `proxy` accepts an HTTP or SOCKS proxy without embedded credentials. * `args` passes additional command-line switches to the selected browser. * `devtools: true` and unpacked `extensions` are unavailable on Firefox and Zen and return an error. Closing a session keeps its profile. The companion closes browsers it launched when it exits, as in [developer mode](/concepts/developer-browser).