Hello, Type '''about:config''' in the address bar and press enter. Firefox displays a warning message to make you cautious about editing the config values. Once you agree, it lists all configuration parameters used by Firefox, which are editable. Now filter for '''browser.warnOnQuit''' and double click (toggle) on it to change the boolean value to true. True means the warning window will be displayed when quitting the browser. To change it, you need to add a new preference to the config file. Right click on the config window, select '''New -> String'''. Enter the preference name as '''general.useragent.override''' and the new User Agent value you prefer.

thank you