As far as I can tell, Firefox is not rendering about:blank under certain conditions. I am using "chrome\userContent.css" to customize about:blank, but sometimes Firefox is producing a blank white screen instead. When opening a new window and sometimes when a new tab is created by closing the last one in a window, this style is not applied. "Tools > Page Info" confirms that the unstyled page is indeed about:blank. Also opening the developer tools to the "Style Editor" or viewing the page source will fix the issue.