in the section named "System Defaults" click on the button named "Make Firefox the default browser".The "Set Default Programs" window will open. In the "Set Default Programs" window, select Firefox from the list of programs on the left and click "Set this program as Default". Then click OK to close the window.How long will Firefox work on Windows 7?
How Long Will Fire Fox Support Windows 7? At minimum until the end of life decided by MS for Win 7, in January 2020. And probably later (depends on APIs needed for new features), like Windows SP2 which is still supported by Mozilla.