I wouldn't get so excited about this. There have been tons of javascript exploits to leak IP addresses in the past, it's more likely that than the FBI running thousands of servers.
Doesn't the tor browser disable javascript entirely?
https://support.torproject.org/tbb/tbb-34/
> We configure NoScript to allow JavaScript by default in Tor Browser because many websites will not work with JavaScript disabled. Most users would give up on Tor entirely if we disabled JavaScript by default because it would cause so many problems for them.
That said, popular websites on/for Tor are free of JavaScript and they have implemented everything (such as captchas, or the "JavaScript is enabled" warning, and so forth) using pure HTML and CSS. They do use OpenResty[1], too, which is NGINX + Lua. Good stuff.
[1] https://openresty.org/, https://github.com/openresty/lua-nginx-module