You can use the Firefox developer tools on your desktop to debug Web sites and Web apps running in other browsers or runtimes. The other browser might be on the same device as the tools themselves or on a different device, such as a phone connected over USB.
The detailed instructions for connecting the developer tools are specific to the runtime.
In the first place, you can connect the developer tools to Gecko-based runtimes like Firefox Desktop, Firefox for Android, Firefox OS, and Thunderbird.
The experimental Valence add-on enables you to debug Web sites in runtimes that are not based on Gecko, like Google Chrome Desktop, Chrome on Android, and Safari on iOS.
Valence is included by default in Firefox Developer Edition.