Hi, thanks for sharing your feedback. This extension can increase CPU load slightly when scrolling through Instagram, but shouldn't trigger any stress on our drive. If you want, you can share a screenshot of you browser's task-manager via my email.
Firefox does support PiP, but doesn't offer a JavaScript API for developers. It only offers its own floating PiP button, but developers cant use this function manually. That's why this feature is currently Chrome exclusive in my extension.