docs: improve README with supported platforms, version formats, and parameters

README.md

@@ -2,9 +2,12 @@
 
 # setup-firefox
 
-This action sets by Firefox for use in actions by:
+This action sets up Mozilla Firefox for GitHub Actions. This action supports the following features:
 
-- downloading and caching a version of Firefox by version and add to PATH
+- Install and set up Firefox onto the runner.
+- Install a specific version of Firefox by the version number or release channel.
+- Cross-platform runner support (Windows, macOS, Linux) and self-hosted runner support.
+- Support for Firefox release channels: stable, beta, developer edition, nightly, and ESR.
 
 ## Usage
 
@@ -38,6 +41,34 @@ jobs:
           ${{ steps.setup-firefox.outputs.firefox-path }} --version
 ```
 
+### Supported version formats
+
+| Version format | Example | Download source |
+| --- | --- | --- |
+| Release channel | `latest` (default), `latest-beta`, `latest-devedition`, `latest-nightly`, `latest-esr` | [Mozilla Releases][] |
+| Specific version | `84.0`, `84.0.1`, `beta-84.0b1`, `devedition-84.0b1` | [Mozilla FTP][] |
+
+[Mozilla Releases]: https://download.mozilla.org/
+[Mozilla FTP]: https://ftp.mozilla.org/pub/firefox/releases/
+
+## Supported platforms
+
+| Linux x64 | Linux ARM32 | Linux ARM64 | macOS x64 | macOS ARM64 | Windows x64 | Windows ARM64 |
+| ---       | ---         | ---         | ---       | ---         | ---         | ---           |
+| ✅        | ❌          | ✅          | ✅        | ✅          | ✅          | ✅            |
+
+## Parameters
+
+### Inputs
+
+- `firefox-version`: *(Optional)* The Firefox version to install and use.
+  Default: `latest`
+
+### Outputs
+
+- `firefox-version`: The installed Firefox version. Useful when given a latest version.
+- `firefox-path`: The installed Firefox path.
+
 ## Contributing
 
 See [CONTRIBUTING.md](CONTRIBUTING.md) for development setup, workflow, and release process.