It's very good. It would be even better if two features could be added. The ability to quickly change proxies with a single click on the toolbar button. Perhaps left clicking or middle clicking there would cycle through the proxies. A keyboard shortcut would be a bonus.
Another feature which was alluded to by Tristan Gallet is the ability to add exceptions i.e. a blacklist of IPs that are not proxied. This is offered in the default firefox implementation of proxying too. For example the ability to configure it so that connections to 127.0.0.1 or your router IP address are not proxied.
Apart from that it's great. Thanks for rewriting it for FF Quantum!