Edit and replay XHR chrome/firefox etc?

Chrome :

• In the Network panel of devtools, right-click and select Copy as cURL
• Paste / Edit the request, and then send it from a terminal, assuming you have the curl command

See capture :

Alternatively, and in case you need to send the request in the context of a webpage, select "Copy as fetch" and edit-send the content from the javascript console panel.

Firefox :

Firefox allows to edit and resend XHR right from the Network panel. Capture below is from Firefox 36:

Chrome now has Copy as fetch in version 67:

Copy as fetch

Right-click a network request then select Copy > Copy As Fetch to copy the fetch()-equivalent code for that request to your clipboard.

https://developers.google.com/web/updates/2018/04/devtools#fetch

Sample output:

fetch("https://stackoverflow.com/posts/validate-body", {
  credentials: "include",
  headers: {},
  referrer: "https://stackoverflow.com/",
  referrerPolicy: "origin",
  body:
    "body=Chrome+now+has+_Copy+as+fetch_+in+version+67%3A%0A%0A%3E+Copy+as+fetch%0ARight-click+a+network+request+then+select+**Copy+%3E+Copy+As+Fetch**+to+copy+the+%60fetch()%60-equivalent+code+for+that+request+to+your+clipboard.%0A%0A&oldBody=&isQuestion=false",
  method: "POST",
  mode: "cors"
});

The difference is that Copy as cURL will also include all the request headers (such as Cookie and Accept) and is suitable for replaying the request outside of Chrome. The fetch() code is suitable for replaying inside of the same browser.