Fuji recently released firmware version 4.20 for the Fuji X-T1 and added several new features and enhancements. According to a new notice from Fuji, however, the firmware also introduced a problem that can “rarely” cause the camera to freeze when shooting in AF-C mode.
As a result, Fuji has pulled the recent version 4.20 firmware from its website and offers a tentative solution for users to upgrade the X-T1’s firmware to version 4.21, which is essentially the same features as rolling back to version 4.10. This means you will lose the new version 4.20 features if you have already upgraded.
Fuji says that it plans on a firmware upgrade in late January 2016 to bring back the features from version 4.20 and fix this bug. you can download the new version 4.21 firmware here on Fuji’s website.