Thanks for the suggestion to use LuCi. I did give that a try. While the normal GL.inet interface yields blank pages (and page source reveals it is trying to load a JS library that Firefox 45 cannot run), LuCi comes up and all the pages at least partly render.

But most pages warn the JavaScript is not running, and many fields, like the ones to choose an access point, do not fill out enough to do anything useful.

Firefox45 is running JavaScript as well as it can. But not well enough to run GL.inet or LuCi.