But even if it was in the US, what you are citing mostly applies to things like cars that you can put aftermarket parts on. They could just say that modifying the firmware is "[using] a product for something other than its intended purpose" and the FTC will just look at you and shrug.
However, if you want to invoke your legal warranty more than 6 months after purchase date YOU have to prove that your modifications didn't cause the defect.
If you are slighted by a foreign company you can still receive a judgement against them or their functionaries, and in extreme matters the company and its distributors would start having issues with CBP and with their banks.
The damages required for such issues can be quite high, $50-75k for federal court filing, but existing precedent would allow you to go after distributors in a local court for a lower amount.
CBP will only get involved if the product is illegal or dangerous.
The solution is to not offer a warranty, though some states do not allow you to disclaim suitability for a particular purpose and establish an implied warranty for items you sell.