Thanks.
I have tried you suggestion and it appears that JavaScript function was not called.
Again it worked under FF and even worked under IE7 if the Report Page (simple HTML page which contains IFRAME) requested directly in browser.
But if the Report Page is requested in one of the frame windows - nothing happened - JavaScript window alert did not appear.
Then I have added a button to the form and called the JavaScript function by clicking the button - it worked, form was submitted and everything was working as intended.
Then I have change "target" attribute of the form which was pointing to IFRAME, thinking that it may confuse things, but it did not help.
It seems the issue is with calling JavaScript from Fusion Charts if the page is requested in one of the frame windows.
Any ideas?
Many thanks.