Have you restarted Firefox after installing that ?

See if it is here : copy/paste about:addons in to your Address Bar and Enter. See if it is under Legacy Extensions as it probably is as is not compatible. Look under Extensions. If is Legacy then click the Button for Find a Replacement.

If not there : copy/paste about:preferences in to your Address Bar and Enter. See if it is at the very bottom of the page Network Proxy

Other than that go back to the Developers page and look for their support info and or FAQ's.

You can look here at Extensions being tracked that are being updated http://bit.ly/2j0Anlj as well the site as a Look For Replacement page.

Please let us know if this solved your issue or if need further assistance.

What helped was "Legacy Extensions" in your solution. "about:addons" opened the Add-on Manager, a page I had already been on. It hadn't occurred to me to click on Extensions. When I did, there was FoxyProxy. It was not Legacy. All I needed to do to configure it was to come out of my stupor long enough for it to occur to me to click on Options.

Thank you very much for your help.