I'm much more worried about something like this in a check-in bag, where the fire can spread uncontrollably. At least in cabin you will notice the fire early and can use the fire extinguisher.
A fire extinguisher is not going to do much against a lithium fire. But at the same time, such fire would end before you even got up from your seat - the only concern would be something inside the plane catching fire, but literally everything inside the cabin is fireproof.
Fire inside the hold is another matter entirely, but there are fire suppression systems fitted into all planes nowadays.
Fire inside the hold is another matter entirely, but there are fire suppression systems fitted into all planes nowadays.