Firefox Javascript Debug doesn't work

Hello everyone,
I am trying using Vs.Php 2.5 on VS2005. I tried a simple application (one button, some JS function, two php files), and tried a debug with the Built-in mode.
I changed the default Apache port to 25500 (debug client port is 7870) and when I run the application with Internet Explorer I can debug on all files (PHP files, Javascript functions in a separate file, say test.js) without problems.

When I set the "Default Browser" (which is Firefox 3.0.3) in the Project properties, I got errors.
In order, after running "Start Debug":

1) A Popup alert in Firefox with title:" Windows - No Disc available" (or not in drive, i'm traslating from Italian title). The message body says something like:"Exception Processing Message c0000013 Parameters 75b1bf7c 4 75b1bf7c 75b1bf7c". Cancel, Retry, Continue?

2) I click on "Continue" and the full page (index.php) is loaded with the button.

3) Clicking on button I can debug only the PHP file called by the button event.

The breakpoints inside the JS script file are "Unbound" and so I cannot debug them in anyway.... is there a way to resolve this? We are evaluating if we can buy it or not, but the plugin MUST debug also on firefox.

Thank you in advance,
Flavio