The data sent by the POST method can be easily altered by the TamperData add-on of the Firefox.
It can be downloaded from the https://addons.mozilla.org/sk/firefox/addon/966/ .
After the installation the Firefox is restarted. TamperData can be accessed inside of the 'Tools' menu.
After its start a window appears. By clicking on the 'Start Tamper' it starts watching for a sent data and asks whether it should be tampered. If so, another window appears showing all the keys and assigned values which can be easily changed.