If you plan for this in advance, you can use the Web Console (Ctrl+Shift+k) to log the request, but you need to right-click a request and turn on Log Request and Response Bodies in order to see the form variables. Then when you click the URL it will show the form data along with the request headers. (Screen shot)

The Firebug extension similarly requires you to open it in advance, but it logs the form variables by default. (Screen shot)

For a more convenient method, perhaps there is another add-on (or a trick I don't know about).

I forgot to mention before... the forum software detected your Firefox as version 18. Is that correct?

Version 18 is not secure; Mozilla discloses security flaws after each new release. Is something holding you back from upgrading to Firefox 24? Please let us know so we can suggest solutions or workarounds.

If Help > About Firefox shows Firefox 24.0, you may need to clear the preference that is misreporting your version number. See: How to reset the default user agent on Firefox.