I am running Ubuntu 18.04.4, and I didn't make any changes to my Firefox settings. A couple of days ago, my Firefox began rendering webpages using a "minimalist background" (see example of how it renders the StackOverflow homepage) that is hard on the eyes and makes some sites unusable.
I have not been able to find a discussion of this issue online. Has anyone else experienced this problem? Is there a simple solution to revert the browser to a factory default or previous settings? I have tried restarting Firefox in safe mode, but the issue was not resolved even then.