Tamper Data no longer works in FF 57+.
This is a re-write that works in FF 57+, however, the webRequest API doesn't currently support changing request body paramters. There are tickets open for both Firefox and Chrome so hopefully it will be resolved sooner than later, but for now this is still very useful for other things:
- Monitoring live requests
- Editing headers on live requests
- Canceling live requests
- Redirecting live requests
- Download and unpack the source code.
- Navigate to about:debugging.
- Click the button to add a temporary extention.
- Select the manifest.json file and hit OK.
It will automatically uninstall when you close the browser.
You can install from the official add-ons page here, here.
Once installed you will see a blue cloud button in the toolbar, to the left of the URL bar. Click that to start tampering.