Combine external javascript

[outlet.ee]

The Firefox'es pagespeed extension suggests to combine external javascript. Rocky's perfomance tips mentions JMerge tool for doing this and I used it. But there are some javascript errors now. I am wondering whether I did it right. Here is what I did:

- made one script called 'global.js' using the tool and uploaded it to .js folder

- deleted the following code prom product.php:

$js_files = array(
   __PS_BASE_URI__.'js/jquery/thickbox-modified.js',
   __PS_BASE_URI__.'js/jquery/jquery.idTabs.modified.js',
   __PS_BASE_URI__.'js/jquery/jquery.scrollto.js',
   __PS_BASE_URI__.'js/jquery/jquery.serialScroll.js',
   _THEME_JS_DIR_.'tools.js',
   _THEME_JS_DIR_.'product.js'
);

- changed the 'header.tpl' by deleting:

>
[removed][removed]
[removed][removed]
[removed][removed]

and changed ....tools.js to ...global.js.

Now I get errors on product page: 'object expected', sometimes also 'tb_pathtoimage is not defined' and 'middle' is not defined.

Has anybody succeeded combining external javascript? How to do it properly?