A few hours ago a vulnerability was discovered in the Tor browser bundle and Firefox web browser. This exploits Windows systems with a high success rate and affects Firefox versions 41 to 50, plus the current version of the Tor Browser Bundle which contains Firefox 45 ESR.
There is no fix for the Firefox browser vulnerability at the time of writing, so it’s recommended that Firefox and Tor browser users temporarily switch to Chrome, Safari or a non-Firefox based browser until an update has been released.
Update now available. https://www.mozilla.org/en-US/firefox/50.0.2/releasenotes/