I got the following instructions from Sony for rolling back the firmware on my X90c but I haven't been able it working past step 8.

1- Make sure you have a USB memory device at hand with at least 1 GB of memory and format it, selecting the FAT32 file system
2- Download the recovery software package and unzip the file
3- Copy the extracted file to the root directory of the formatted USB memory device
4- Turn on the TV set
5- Insert the USB memory device into the USB slot located at the side of the unit
6- Pull out the power plug and wait for at least 10 seconds
7- Reinsert the power plug
8- Press the key to turn the unit on and confirm that the LED under the SONY logo at the bottom of the TV set turns on in green
9- After 20 to 30 seconds, confirm that the LED starts blinking in amber color. LED blinking indicates that the firmware update is installing
10- After 1 to 2 minutes, the LED should stop blinking. Press the key to turn on the TV
11- If the SONY logo appears on the screen and the LED turns on in green, the recovery software installation was successful.
12- Pull out the USB memory device from the TV
13- Finally, we recommend updating to the latest firmware for your Bravia TV by going to this page and following the update instructions

IMPORTANT: During the firmware installation process, do not remove the USB device, switch off the TV set or remove the power plug.

Below is the link for downloading the recovery software:

download.sony-europe.com/pub/tv/ITC3_RECV_STEU1.ZIP

 

Please post in this thread if you manage to get it working.