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

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)}

Yes, there are two I know of:

• Tamper Data

• Modify Headers

• RESTClient : request oriented and user-firendly (runs in a tab, XML answer coloration)

  Development version (works with Firefox 4.b9) here :

  https: //github.com/chao/RESTClient

• Poster : handling binary data POST (with file loading)

  https: //addons.mozilla.org/fr/firefox/addon/poster/