Yesterday morning, August 5, a Firefox user informed us that an advertisement on a news site in Russia was serving a Firefox exploit that searched for sensitive files and uploaded them to a server that appears to be in Ukraine. This morning Mozilla released security updates that fix the vulnerability. All Firefox users are urged to update to Firefox 39.0.3. The fix has also been shipped in Firefox ESR 38.1.1.
Head to the Facebook Official Group the get more info about the updates.