Many Tor users are not technically savvy, and JavaScript is commonly used with HTML5 in modern web sites. The only reason the Tor project allows JavaScript to be on by default in the Tor browser is usability. JavaScript), then they may be hard pressed to find a viable exploit even if they have access to zero days etc. If the vectors for these zero-days are disabled (e.g. the FBI), they have access to zero-day exploits. In the case of a serious adversary like a state-backed entity (e.g. In general, enabling JavaScript opens the surface area for many more potential attacks against a web browser. You can read more about it in Ars Technica. This caused the computers to call back to an FBI server from their real, non-anonymized IP, leading to the deanonymization of various users.

The FBI kept servers online, and then installed javascript paylods which exploited a zero-day exploit in Firefox.

The first major incident where this happened was with the 'Freedom Hosting' seizure by the FBI. There are a number of known vulnerabilities, that have been used, to deanonymize Tor users via leveraging JavaScript.

Disable javascript tor browser mac