Unfortunately I can't submit this via your bug report system, since I keep getting an error message.

In Firefox 55.0.2 64-bit, when loading certain webpages (including http://mybrowseraddon.com/dark-mode.html) the browser displays an error message saying that the webpage is slowing down your browser. This happens all of the time on certain webpages (mybrowseraddon) and only some of the time on other webpages (duckduckgo.com). I am using Windows 10 Pro 64-bit.