Use of ES6 "class" not supported by Pale Moon browser.

[BenFenner]

The Pale Moon browser is one of the browsers we use at our web development shop, and is an important browser for us to test against. It does not yet (may never?) support the ES6 keyword "class" which is used in the file performance_bar.js of this project. Including peek bar in our Rails projects cause JS to fail to work in many confusing ways while using the Pale Moon browser. Please consider supporting this web browser. Until that happens, we can't use the peek plug-in, and instead have to use an alternative. =(

Additional info:
https://forum.palemoon.org/viewtopic.php?f=13&t=17663

[glaszig]

same for ie11: