[PSA] If you’re using Firefox, particularly on Windows, stop doing that.
Sad (Red) Panda needs to be put in quarantine for a while.
Firefox (versions 41 – 50, which includes Tor Web Browser (Firefox 45 ESR)) currently has a 0-day exploit. This means there is a remotely-accessible and exploitable security concern with the application for which there is currently no fix, and the internet at large is actively taking advantage of the situation. This will undoubtedly be fixed, but until it is, you should switch browsers.
You can learn more about the situation from Wordfence (which is where I originally learned of it), Ars Technica, The Register, and any number of other web sites. Just, you know, use a non-Firefox browser when you start poking around looking for information.