I managed to make it work in 57, and I think it's the best proxy switcher here. Other extension just throw premium proxies in your face and collect your data for promotional needs.
Firefox proxy settings were interfering with FoxyProxy. Switched to "Use system settings" and it's works fine.
I know developer is working hard to make it work in new firefox.
There was a good feature in old version. Adding sites to list by hotkey. I hope you bring that to new version eventually.

After the FF57 update I imported my configuration from FF56. I thought it was working fine, the URLs I had set up patterns for were being fetched through the specified proxy, or so I thought because the logs told me so. However this morning, I forgot to start Tor (the proxy I'm using), however all the URLs were being loaded, even the ones that were, according to the FoxyProxy log, loaded through the proxy. So it seems like it isn't working at all.

Great Add-on however! I understand the difficulties with porting the Add-on to WebExtensions and I am confident that the developer is capable of fixing this.