Unless you happen to live in a jurisdiction that care more about users than companies, like the EU. The manufacturer would have to prove that the new custom firmware is actually the cause of the damage, otherwise they need to fulfill the warranty guarantee regardless of what firmware you run.
> Good luck proving that changing the firmware is not voiding a warranty.
You're thinking about it the wrong way around. The manufacturer has to prove that the custom firmware is the reason it broke, you don't have to prove anything. Username not accurate.
Unless you happen to live in a jurisdiction that care more about users than companies, like the EU. The manufacturer would have to prove that the new custom firmware is actually the cause of the damage, otherwise they need to fulfill the warranty guarantee regardless of what firmware you run.