Update: Mozilla now released Firefox 50.0.2 to fix this issue, and the tor project released the corresponding tor browser 6.0.7.
Mozilla is working on a patch. And Tor Browser 6.0.7 which will hopefully be released later today is scheduled to include the fix.
Until then: Best not to use Firefox if you can help it. The current version of Firefox, 50.0.1, which was released on Monday, is still vulnerable. While the exploit hasn't been spotted yet outside of Tor, expect it to show up by the time you read this.
An analysis of the shell code used in the exploit by "Wack0" concluded that the code is very similar to an exploit used by law enforcement in 2013 against an older version of Firefox/Tor Browser. 
Nov 30th 2016
1 year ago