Great add-on, my constant companion for years. But with the latest Firefox changes, 2 of its most useful (to me) features - CSS > Display Style Information and Information > View document size - are gone. Workaround! What I did was: a) Downloaded and installed Waterfox (I have v.55), then (b) installed in Waterfox an older version of Webdeveloper from the URL Chris gave: https://addons.mozilla.org/en-US/firefox/addon/web-developer/versions/ I got Version 1.2.13. (c) Make sure you set Waterfox updates to "let you choose whether to install them" and set the webdeveloper add-on to "Automatic updates: off", otherwise you might launch Waterfox one day and find you've got webdeveloper 2. Adding an add-on to Waterfox is the same as for Firefox - you'll get a message saying "add to Firefox", but if you're doing it from Waterfox, it adds it to Waterfox. All of this is just to keep "the old webdeveloper" working, cannot vouch for it being safe.

Oh and... be careful. Waterfox just advised me to update to the new "responsive design" module (or whatever), which I did. And webdeveloper lost half of its tricks. Will have to restore the system, I guess.