I tried three method, and none of them worked. 1. I uncheck the pop-up 2. the box under option/advanced of checking it as default browser is uncheck 3. the about:config browser.shell.checkdefaultbrowser was set to false. but every time I open firefox the check still pops up. which is very annoying!! also when I close the window with multiple tab warning, even I unchecked it, it still pops up the warning next time. the OS Windows 8