WitchBlock — Firefox Extension

Strips server-side stitched ads from Twitch HLS playlists by injecting a fetch hook into the Amazon IVS WASM worker before it initializes.

How it works

The IVS worker fetches an M3U8 playlist every few seconds. Ad breaks are signaled by #EXT-X-DATERANGE markers with Twitch-specific attributes. We prepend a self-contained IIFE to the worker script that wraps self.fetch, reads each M3U8 response, strips ad segments, and returns the cleaned body.

Development journey

Goal

Block Twitch stitched (SSAI) ads. Twitch injects ads directly into the HLS stream as extra video segments, marked with custom M3U8 tags. Removing those tags and their associated segments before the player sees them means no ad plays.

Attempt 1: Tampermonkey

Tampermonkey scripts run in the page's JS context. The natural approach is to hook window.fetch and intercept M3U8 responses.

Why it fails: Twitch streams through Amazon IVS, which runs inside a dedicated Web Worker (amazon-ivs-wasmworker.min-*.js). Dedicated workers have their own global scope (self), completely separate from window. A Tampermonkey hook on window.fetch never touches the worker's self.fetch. There is no way for page-context JS to reach into a dedicated worker's scope.

Attempt 2: Firefox extension with filterResponseData on ttvnw.net

Firefox MV2 extensions can use browser.webRequest.filterResponseData to intercept and rewrite response bodies. The plan was to intercept M3U8 responses from *.ttvnw.net (Twitch's CDN) and strip ad segments.

Why it fails: Firefox's webRequest API does not expose network requests made from inside dedicated Web Workers. The worker's fetch() calls to ttvnw.net are invisible to the extension's onBeforeRequest listener. Confirmed by adding a debug listener on <all_urls> — requests from assets.twitch.tv (page-level script loads) appeared, but zero requests from ttvnw.net (worker-level M3U8 fetches) appeared.

Attempt 3: Intercept the worker script itself

Key insight: while the worker's fetches are invisible to webRequest, the worker script file is fetched by the page and is visible as a normal script or other type request from assets.twitch.tv.

The IVS worker captures self.fetch at module initialization time:

T = self.fetch.bind(self)

If we prepend our hook IIFE to the worker script before that line runs, T becomes our hook. The worker then uses our patched fetch for all its M3U8 requests for the lifetime of the stream.

Implementation:

1. Listen for *://assets.twitch.tv/assets/amazon-ivs-wasmworker* in onBeforeRequest.
2. Use filterResponseData to buffer the full worker JS response.
3. In onstop, prepend the hook IIFE (encoded as UTF-8 bytes) before the original script bytes.
4. Write the patched buffer and call filter.close().

Bug discovered: The WASM binary (amazon-ivs-wasmworker.min-*.wasm) has the same URL prefix and also matched the listener pattern. The filter was buffering and rewriting the binary WASM file, corrupting it (wasm validation error: at offset 4: failed to match magic number). Fix: guard the listener with if (!details.url.includes('.js')) return; so WASM and other assets pass through untouched.

Result: Working. The hook intercepts every M3U8 poll, strips ad segments, and the player receives clean playlists. Confirmed output:

[TwitchAdBlock] ad stripped — removed 8 segments from https://aps31.playlist.ttvnw.net/...

The player detects a gap where ad segments were removed and jumps over it (jumping 14.116s gap), returning to live content immediately.

What is NOT blocked

CSAI (client-side ad insertion) is a separate system. Twitch makes a request to edge.ads.twitch.tv to fetch ad metadata and plays a client-side ad independently of the HLS stream. This extension does not block CSAI. A CORS error for edge.ads.twitch.tv in the console is Twitch's CSAI request being blocked by uBlock Origin (which was active during testing), not this extension.

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Why this approach cannot work in Chromium (Chrome, Brave, Edge)

Chrome deprecated Manifest V2 and requires extensions to use Manifest V3. The critical difference:

Capability	MV2 (Firefox)	MV3 (Chrome)
Modify response body	filterResponseData	Not available
Block requests	webRequest blocking	declarativeNetRequest

MV3's declarativeNetRequest can block or redirect requests but cannot modify response bodies. There is no equivalent to filterResponseData in Chrome MV3. Without response body modification, there is no way to intercept the worker script and prepend code to it.

Firefox continues to support MV2 and filterResponseData, which is why this approach is Firefox-only.

A Chromium-based solution would require either:

• Forking Chromium/Brave and patching the browser itself to add response-body interception at the network layer.
• A proxy (e.g., mitmproxy) sitting between the browser and Twitch's CDN, which intercepts and rewrites M3U8 responses at the network level. See ../twitch/mitmproxy/ for that approach.

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Files

firefox-twitch/
├── manifest.json       MV2 manifest
├── background.js       worker hook IIFE + onBeforeRequest listener
└── icons/
    ├── icon-48.svg     extension manager icon
    └── icon-96.svg     high-DPI extension manager icon

Install (temporary, for testing)

1. Open about:debugging
2. Click "This Firefox"
3. Click "Load Temporary Add-on"
4. Select manifest.json

Install (permanent, signed)

Firefox requires extensions to be signed by Mozilla unless you use Developer Edition or Nightly with xpinstall.signatures.required set to false in about:config.

To get a signed .xpi:

1. Create an account at addons.mozilla.org
2. Install web-ext: npm install -g web-ext
3. Package and submit for signing:

   web-ext sign --api-key=<AMO_JWT_ISSUER> --api-secret=<AMO_JWT_SECRET>
   

4. AMO returns a signed .xpi you can distribute or install directly.

Alternatively, list it publicly on AMO and users can install it from there.

Note on Manifest Version

This extension uses Manifest V2, not V3. That is intentional. browser.webRequest.filterResponseData (response body rewriting) only exists in MV2. Chrome removed it in MV3, which is why this extension is Firefox-only. Firefox continues to support MV2 indefinitely for this reason.