Only JavaScript can be used as a client-side scripting language (and VBScript too, in IE). The attribute srcsimply indicates some kind of URL, and that URL will return JavaScript.
So, the URL http://www.yellowbridge.com/lib/yellowbridge.dll really returns JavaScript. If you save its contents as a local text file, you can read the script.
Without knowing anything about the site or its JavaScript, I would suggest that they dynamically generate some part of the script file from the DLL.
: , JS, , script .