Cross-browser builds

ExtForge produces a separate output directory for each declared browser. The same source compiles to dist/chrome/, dist/firefox/, dist/edge/, and dist/safari/ — each with its own manifest.json generated from your shared config plus per-browser overrides.

Declaring browsers

In extforge.config.ts:

import { defineConfig } from 'extforge';

export default defineConfig({
  browsers: ['chrome', 'firefox', 'edge', 'safari'],
});

extforge build iterates this list. extforge dev defaults to chrome; use --browser firefox to run the watcher against a different target.

Valid values: chrome, firefox, edge, safari. Duplicates are deduplicated.

Per-browser manifest overrides

Use manifest.browserOverrides to merge browser-specific values into the generated manifest. Overrides are shallow-merged over the base manifest fields.

manifest: {
  background: {
    entrypoint: 'src/background/index.ts',
  },
  browserOverrides: {
    firefox: {
      firefoxId: 'my-extension@example.com',
    },
    safari: {
      // safari-specific keys here
    },
  },
}

Why Firefox uses `scripts` and Chrome uses `service_worker`

Manifest V3 specifies background.service_worker for Chrome. Firefox MV3 (109+) supports both background.service_worker and background.scripts. ExtForge writes the correct key per-browser automatically:

Browser	Background manifest key
Chrome	`background: { service_worker: "..." }`
Firefox	`background: { scripts: ["..."] }`
Edge	`background: { service_worker: "..." }`
Safari	`background: { service_worker: "..." }`

You declare a single background.entrypoint in config. The manifest generator reads the browser capability matrix and outputs the appropriate key. No override needed.

If you need Firefox to use service_worker explicitly (not recommended), you can override via browserOverrides.firefox.background.

The compat checker

ExtForge scans your entry-point source files for chrome.* and browser.* API calls and cross-references them against a committed slice of MDN browser-compat-data (at src/core/compat/data.json). It reports which APIs are unsupported by which of your declared browsers.

By default, compat issues are warnings. Pass --strict to the build command to treat them as errors:

extforge build --strict

In extforge.config.ts you can make strict mode permanent for dev:

export default defineConfig({
  dev: { strictCompat: true },
});

Per-line suppression

Suppress a single compat warning with a comment immediately above the call:

// extforge-ignore-compat: Chrome-only; Firefox uses the popup fallback
chrome.sidePanel.open({ tabId });

The reason string (after the colon) is required. A bare // extforge-ignore-compat without a reason is rejected — the checker logs a warning and does not suppress.

The suppression applies to the next non-blank, non-comment line. Blank lines and other comments between the suppression and the call are skipped.

Common pitfalls

`chrome.tabGroups.*` — not on Safari

chrome.tabGroups is Chrome/Edge only. Safari does not implement it.

// Wrong — breaks on Safari
chrome.tabGroups.query({}, (groups) => { /* ... */ });

// Right — guard or drop Safari from browsers[]
if (typeof chrome.tabGroups !== 'undefined') {
  chrome.tabGroups.query({}, (groups) => { /* ... */ });
}

Or, if you need tab groups, drop Safari from your browsers list:

export default defineConfig({
  browsers: ['chrome', 'edge'], // no safari
});

`chrome.sidePanel.*` — Chrome and Edge only

chrome.sidePanel is not available on Firefox or Safari. Provide a fallback popup for those browsers.

// extforge-ignore-compat: Chrome/Edge only; Firefox gets a popup via browserOverrides
chrome.sidePanel.open({ tabId: tab.id! });

In config, provide a Firefox fallback:

manifest: {
  sidePanel: {
    defaultPath: 'src/ui/sidepanel/sidepanel.html',
  },
  browserOverrides: {
    firefox: {
      // Firefox sees a popup instead
      action: {
        defaultPopup: 'src/ui/sidepanel/sidepanel.html',
        defaultTitle: 'Open panel',
      },
    },
  },
}

`chrome.declarativeNetRequest.*` — partial on Firefox and Safari

declarativeNetRequest is in MV3 for Chrome, Edge, and Firefox, but the exact API surface differs. Safari support is partial. Check src/core/compat/data.json for the specific method coverage. Gate on typeof chrome.declarativeNetRequest !== 'undefined' or audit with extforge build --strict.

Background: service worker vs. scripts

As noted above, ExtForge handles the service_worker / scripts split automatically. You only need a browserOverrides entry if you want to supply different background entry points per-browser, not just different manifest keys.

Loading per-browser builds

After extforge build, the output structure is:

dist/
  chrome/
    manifest.json
    background.js
    popup.html
    ...
  firefox/
    manifest.json
    background.js
    popup.html
    ...
  edge/
    manifest.json
    ...
  safari/
    manifest.json
    ...

Load the extension from dist/chrome/ in Chrome’s extension manager, dist/firefox/ in Firefox, and so on.

In CI, build all browsers in one command:

extforge build
# or build a single browser:
extforge build --browser firefox

See deployment for packaging and store submission.

See EXT_COMPAT_UNSUPPORTED for the error reference when strict mode is on.