If it's all plugged in correctly do you have another hdmi cable to try on ps5? Wondered if plugging in the firestick moved a dodgy cable.
You can try removing the firestick, does it return to normal? If ps5 still bad swapping over the ps5 and firestick on TV as an experiment to rule out TV port.
Lastly make sure TV and console are on latest firmware. Read Samsung had a problema few years ago that affected hdmi handshake with multiple sources plugged in causing blackout.
Cable is deffo fine, as it was being swapped between ps5 and virgin box up till last week, when plugged into virgin box it was perfectly fine, ps5 not.
Removing the firestick does solve the problem, so I suspect it’s like you say an issue with the handshake, as it is indeed a Samsung. Did you see anywhere if it was fixed? All three are fully up to date.