According to Fujifilm, “We have found a malfunction in the firmware ver.4.20 launched on December 17 2015, which rarely causes freeze when shooting in AF-C mode. So we will suspend the disclosure of this firmware right away.” So if you haven’t updated yet, you probably should hold off on the update.
For those who already updated, Fujifilm has released a new update. This is an odd update because while the version goes to 4.21, in reality this is the same firmware as before except with a different version number. Fujifilm writes, “As a tentative solution for customers who have upgraded their cameras to firmware ver.4.20, we have prepared the new firmware version of 4.21 whose functions are same as the previous version of 4.10.”
Of course users don’t have to update to 4.21 if they’re not experiencing any issues with 4.20, but if you’d like to be safe then you probably should. According to Fujifilm, they expect that the 4.20 firmware will be fixed towards the end of January 2016, so you’ll want to keep an eye out for it then.