I am back here again! :(
Heeding to your advise, I removed all the code that rewrites the URLs of static content on my website to another domain. Instead, like Joost suggested, I am using W3 Total Cache's Generic Mirror CDN type to do the job.
Problem is, everything's fine, except for two plugins whose JS and CSS files are still being served from the same domain as the one that serves WP. Basic Comment Quicktags is one. Any idea why?