Responsive Inspector Reloaded allows inspecting page media queries. Same as original, but it works on modern browsers.
Original version of Responsive Inspector was broken.
Piotr Walczyszyn (developer) stopped supporting and updating of extension. This version is open sourced and have GitHub project page: https://github.com/MistaTwista/Responsive-Inspector-Reloaded
Feel free to fork and making pull requests or add issues.