Not just in a single thread, in the same thread as DOM rendering. It's conceptually impossible to use a website in the time the javascript executes (except for webworkers etc).

Still, javascript issues tend to cause too many hangups of other parts (browser UI or other websites). We are in the awkward position where OS threads are considered too heavyweight to use an entire thread for each tab, but browsers haven't implemented a more lightweight alternative either. So things just share threads when they really shouldn't.