The latest official navcore for the GO 910 is indeed 7.903 and has been for a while.
You are confusing the term firmware with navcore. There is no firmware in TomToms. The only thing that comes close to firmware is the bootloader, but that's irrelevant for your device. Navcore is like an operating system on a computer, such as Windows. The bootloader is like the BIOS in a computer.
Either use the official 7.903 navcore which is the most reliable, or experiment with other navcores, such as 9.051.
Updating the navcore through TomTom Home then deleting it doesn't achieve anything. The way to install a later, modified navcore is simply wipe the device clean, then copy paste all the folders and files of the navcore into the TomTom drive.
It is recommended to backup your TomTom using Windows Explorer copy paste before proceeding.