Thankfully, I stumbled upon a post in the Foscam forums that provided a working version of the plugin. However, I recently purchased a third camera (model FI9828P V2), and its plugin didn’t work with macOS Sierra. I’ve had two Foscam security cameras (model FI9805W) for a few years, and I’ve been able to view their image and adjust their settings with no problems using the official plugin in Safari, which worked fine on Sierra too.