Dear Firefox Staff: I downloaded the updated firefox 39.0. I did Not have this problem with my prior version. With this 39.0 version, I have the following problem: There is a problem with changing the Background Color of websites. When I go to the top-left of the screen and click on "Firefox", in the dropdown menu, I click on "Preferences:", then it pops up a new Tab (it used to just pop up a mini-window). In any case, in that new Tab, I then click on "Content:", then I click on "Colors:", at which point it opens a mini-window. in that mini-window, I then next to "Background:" I selected the background color Gray. (i.e. I want the background of all websites to be gray, so it's easier on my eyes to read, especially in a darker room, as my Macbook Pro's Contrast (sun) button does not go dark enough, and thus I typically rely on Firefox's Background Preferences to make the screen less bright). Anyways, after selected the background color Gray, I then click on (i.e. select/checkmark) "Use system colors". At this point, using the older version of Firefox, the backgrounds to all websites would become Gray. This does NOT happen with this new version of Firefox; the Background color remains the same (i.e. defaults to the typical website's own preferences; it does not become Gray like i want it to). I have tried refreshing the pages also, and it still does Not work. Please fix this Background Color, (for this new version of Firefox, and future versions), such that I can make the Website Backgrounds Gray. Thanks so much!