GreyBackgrounds reformats all HTML pages to be easier to read by making the backgrounds gray and text black. All fonts are changed to the default sans-serif font. You can change the default sans-serif font in Firefox via Tools > Options > Content > Fonts & Colors > Advanced > Sans-Serif. From that panel you can also change the Minimum Font Size so that fonts will not be too small, 11 is usually what I set it to.
To turn GreyBackgrounds on, check the menu entry... View > GreyBackgrounds, or right-click anywhere on the page and check GreyBackgrounds. Uncheck either to turn it off.