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.
However, I recently purchased a third camera (model FI9828P V2), and its plugin didn’t work with macOS Sierra.
Thankfully, I stumbled upon a post in the Foscam forums that provided a working version of the plugin. I took the liberty of uploading to my blog as well, you can download it from here (here you can find an older version).
UPDATE: The plugin still works with macOS High Sierra, but I found a newer version that works with more cameras, including the FI9851P V3 which I recently purchased.