catch an event in the overlay.xul/js

[io]

Hi, i have done an toolbar for mozilla...overlay.xul (and ovrelay.js);
I would like to run a function "carica()" at every page that is loaded
in the browser...
i have try with

set collapsed(aValue)
{
if (aValue && this.mProgressBar.value != 100)
{
this.mProgressBar.value = 100;

}
carica();
},

but this function is loaded two times at every refresh and one time at
every tab opened....