How do I POST to a web page using Firebug?
I know this is an old question, but I recently stumbled upon the same problem and wanted to share the method I am using.
Assuming the web site you want to POST to has a form with method="POST" (a very likely scenario), you can use Firebug's JavaScript command line to programmatically submit a POST request. Just click the "Show Command Line" icon in Firebug and enter something like this in the narrow text box at the very bottom of the window:
document.forms[0].submit()
Maybe this helps someone.