Unlike my post Wednesday, this is one you should do Right Now(TM) if you have Firefox installed and aren't getting automatic updates. And even if you're getting updates automatically, you should check your version if you haven't updated since Wednesday. This vulnerability is being actively exploited in the wild.
The latest version is 72.0.1; you can check this by choosing the "About" item on the "Help" menu. The corresponding Android version is 68.4.1; "About" is the last item on the "Settings" menu. The update doesn't appear to be necessary on iOS (presumably because it's using a different just-in-time (jit) compiler)-- version 20.0 was released back in October.