[firefox] Outputted manifest is in v2 format despite manifest_version set to 3

[dceluis]

Describe the bug

Creating a new project with manifest-v3 and building the extension for firefox outputs a manifest with an incorrect v2 format

import { defineConfig } from 'wxt';

// See https://wxt.dev/api/config.html
export default defineConfig({
  manifest: {
    manifest_version: 3,
    name: "Test repro",
    version: "1.0",
    description: "Description",
    web_accessible_resources: [
      {
        resources: ["images/*.png"],
        matches: [ "<all_urls>" ]
      }
    ]
  },
});

// .output/firefox-mv2/manifest.json
{
    "manifest_version":3,
    "name":"Test repro",
    "description":"Description",
    "version":"1.0",
    "web_accessible_resources":["images/*.png"],
    "background":{"scripts":["background.js"]}
}

This is problematic because manifest_version is set to 3 while the web_accessible_resources key is formatted as per v2.
This leads to not being able to import the extension in Firefox, as it is expecting a true v3 manifest structure.

Reproduction

#!/usr/bin/env bash
mkdir wxt_tmp_repro

cd wxt_tmp_repro

# Initialize the Bun project (creates package.json with default content)
bun init -y

# Install wxt as a dev dependency
bun i -D wxt

# Insert the "scripts" block after the "name" property in package.json.
# This uses sed to append the block immediately after the line matching "name":
sed -i '/"name":/a\
  "scripts": {\
    "dev": "wxt",\
    "dev:firefox": "wxt -b firefox",\
    "build": "wxt build",\
    "build:firefox": "wxt build -b firefox",\
    "zip": "wxt zip",\
    "zip:firefox": "wxt zip -b firefox",\
    "postinstall": "wxt prepare"\
  },
' package.json

# Create the entrypoints directory if it doesn't exist and add the background script.
mkdir -p entrypoints
cat > entrypoints/background.js << 'EOF'
export default defineBackground(() => {
  console.log('Hello world!');
});
EOF

# Create the wxt.config.ts file with the provided content.
cat > wxt.config.ts << 'EOF'
import { defineConfig } from 'wxt';

// See https://wxt.dev/api/config.html
export default defineConfig({
  manifest: {
    manifest_version: 3,
    name: "Test repro",
    version: "1.0",
    description: "Description",
    web_accessible_resources: [
      {
        resources: ["images/*.png"],
        matches: [ "<all_urls>" ]
      }
    ]
  },
});
EOF

# Build for Firefox
bun run build:firefox

# Output the generated manifest.json to the console
cat .output/firefox-mv2/manifest.json

Steps to reproduce

Run the above repro as bash repro.sh

System Info

System:
    OS: Linux 6.12 Arch Linux
    CPU: (16) x64 AMD Ryzen 7 5700U with Radeon Graphics
    Memory: 10.14 GB / 14.45 GB
    Container: Yes
    Shell: 5.9 - /usr/bin/zsh
  Binaries:
    Node: 23.7.0 - /usr/bin/node
    Yarn: 1.22.22 - /usr/bin/yarn
    npm: 11.1.0 - /usr/bin/npm
    bun: 1.1.43 - ~/.local/share/mise/installs/bun/1.1.43/bin/bun
  npmPackages:
    wxt: ^0.19.26 => 0.19.26

Used Package Manager

bun

Validations

• Read the Contributing Guidelines.
• Read the docs.
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• The provided reproduction is a minimal reproducible example of the bug.

[aklinker1]

I can add a warning and make sure to ignore manifest.manifest_version. But basically, you're not setting the manifest version in the correct location. You should set manifestVersion instead:

export default defineConfig({
+ manifestVersion: 3,
  manifest: {
-   manifest_version: 3

I'm surprised TS let you define that key, I thought the type excluded it.

https://wxt.dev/api/reference/wxt/interfaces/InlineConfig.html#manifestversion

[dceluis]

That explains why the final manifest.json was being written to mv2 folder AND came out with a manifest_version: 3.

Thanks. A warning might be sufficient, yes.