One of the biggest improvements on the XH-1 firmware is that videos over 4GB can now be recorded as a single file (provided a memory card of 64GB or higher is used). The in-body stabilization has also been improved from 3-axes to 5-axes, and when used with current and updated lenses, provides over five stops of image stabilization based on CIPA standards.
Most of the updates to the X-T3 improve video, including 4K HDR recording in Hybrid Log Gamma (HLG), and simultaneous output of a film simulation and F-log footage, allowing you to record in the log format while checking the film simulation image on an external monitor.
The 4GB file limit on recording has not been corrected with this firmware for the XT-3, though it is expected to be announced on a coming update in January.
See links below for the firmware update downloads and full details on the updates.
X-T3 Firmware Update – Download Here
X-H1 Firmware Update – Download Here