I've tried changing my GTK theme, and starting with a chosen GTK theme that doesn't have black background. I've tried setting colors in Preferences. I've tried disabling my all-ons. I've tried doing each of these things, shutting down Firefox, and restarting it. I'm on Ubuntu 16.04.

Where is it getting the background color setting and how do I change it? I know that it uses a hard-coded black text color on error pages, that can't be changed, while using a background color setting from somewhere that might also be set to black, and that this bug has gone unfixed for years, but I don't know where it's getting the background color.