Mozilla has announced that it's in the process of patching Firefox after discovering an exploit out in the wild "that searched for sensitive files and uploaded them to a server that appears to be in Ukraine." The vulnerability was discovered in an ad on an unnamed Russian news site on Wednesday, although it's not clear where else it might have appeared.
So, right now would be a great time to update your browser. "The exploit leaves no trace it has been run on the local machine," Mozilla says. "If you use Firefox on Windows or Linux it would be prudent to change any passwords and keys found in the above-mentioned files if you use the associated programs."