V@no, Thanks a million
Its been two days now and I was trying everything, looking into page_header.php and function.php files, finding some clue that could solve the issue.
Mainly I was looking for this line and trying to get some idea. I believe this calls for redirect.
header("Location: ".$site_sess->url($url, "&"));
But I guess I was looking in wrong place with wrong code.
Just incase, if someone is interested (got the same issue) ,
I replaced
if (strstr($url, 'index.php')) {
$url = str_replace('index.php', '', $url);
With
if (strstr($url, 'index.php')) {
$url = str_replace('index.php', './', $url);
in include/sessions.php file and now i am no more getting blank page with FF and Opera. (Both now redirect to main page)
Once again, Thanks for helping me out.