This is a great extension that I've used for years. Unfortunately FF 57 has partly crippled it. I'm keeping my FF at 56 and installing an prior version of Web Developer until FF fixes its API (which it probably won't). But I really want to thank chrispederick for his great work on this and updating it for FF 57 as best as possible.
I have a dozen critical extensions and 9 of them are still "Legacy" with no suitable replacements. Unfortunately all of the three that were updated have lost functionality I need. So I'm guessing that's why the others aren't updating because there's no way to do it without breaking what they're trying to do. So looks like FF 56 Forever. Or if I'm going to have to lose all my extensions, I might as well look for a new web browser too. FF is no longer going to have real useful extensions anymore. Which is sad as I've been using FF since the days of the Mozilla browser.