cURL is not part of Firefox or Firebug, i.e. it cannot be executed there. As the Firebug wiki explains:
cURL is a command line tool for transferring data with URL syntax. Firebug's Copy As cURL command recreates the HTTP request (including HTTP headers and query string parameters) and copies it as a cURL command string to the clipboard. The string can be pasted into a terminal window to execute the same request or for example pasted to a client that supports cURL. In cases where the server serves compressed responses the preference
extensions.firebug.net.curlAddCompressedArgument
can be set totrue
and Firebug will add the--compressed argument
to the generated command string.
It also refers to the cURL homepage.
What Firebug offers is to resend the right-clicked request via the Resend option within the context menu.