Open Firefox web browser and drag your cursor towards ‘Tools’.
Click ‘Tools’ and select ‘Options’ from the drop down menu.
A small window will appear on your screen where you can see the option of ‘Content tab’. Below ‘Content Tab,’ Java Script is enabled. You have to uncheck it in order to disable it.
Now in a new tab type www.yahoo.com in the address bar and press enter.
Log in to your Yahoo account by entering your password and email address.
Before the page loads and you have access to your account, a message will be displayed on your screen saying Java Script is required in order to use new Yahoo Mail. Two options will be visible on your screen; you will have to choose the first option which says ‘Return to a Previous Version of Yahoo! Mail.’
By following these steps you will switch your mail version back to Yahoo Mail Classic. But before you access other websites, make sure you have enabled Java Script or else browsing will not be done efficiently.