Are there Firefox extension (or any other browser) that allow to send arbitrary POST data to a webpage? [closed]

Answer 1

Try Fiddler. From their own documentation:

"The Request Builder allows you to craft custom requests to send to the server. You can either create a new request manually, or you can drag and drop a session from the Web Sessions list to create a new request based on the existing request."

_{(source: ifinity.com.au)}