The <postexecution><programs><program> element needs to have a pathtype= and a path= attribute specified.
Also, verify that the Firefox Setup 10.0.2.exe is returning only the '0' successcode in all instances. If the installer is generating a Reboot Required state, it will return the '3010' successcode, and you will need to explicitly define that in the <execution><programs><program> element, or else the failureaction="stop" attribute will prevent the execution of the entire <postexecution> block.
How can i determine the "path" ? This exe file is inclued in my package.
<program type="exe" operation="runandwait" pathtype="simplepath" path="???" name="post_script.exe" successcode="0" failureaction="stop" enabled="true">
Thanks in advance