First, this isn't a free service. I bought a lifetime membership and have been able to get the app to work on my smartphones, but the browser extension has never worked for me. So don't bother, even if you're a paid subscriber.

The Windows app sometimes works but I was hoping to get this Firefox plugin to work so that I could set the VPN at the browser level only rather than for all incoming/outgoing connections on my computer.

It's been 13 months (March 2019) since they launched this plug in. It's never been updated or had any bug fixes.