Same issue. Latest Thunderbolt Firmware update made my HDMI output unusable. I can not connect to an external monitor. We need a fix ASAP! Dell technical support was unable to help so far...
Thanks for posting.
Apologies for your system not working as expected.
Before you updated the Thunderbolt Firmware, were you able to upgrade the Intel Thunderbolt Controller Driver to A01 before upgrading the Intel Thunderbolt Firmware?