Q: Is the iPod Nano 7th Gen firmware upgradeable ?

iPod nano's onboard software can be updated.  But "upgrade" implies significant improvements to features.  Unlike an iPod touch, which is an iOS computing device that is designed to support new capabilities (from third-party apps and upgraded iOS features), iPod nano is designed primarily as a media player.  Its hardware is designed and optimized specifically for its built-in features, nothing more or less.

Apple may update iPod nano's onboard software (as needed) to fix bugs and make other minor changes, but Apple is unlikely to upgrade it with significant changes to built-in features.  What it does when you buy it is what it does for its usable lifetime.