These can help you out:

Delete browsing, search and download history on Firefox

How to clear the Firefox cache

Thanks your help but it did not work. Yahoo main page is not like on IE, it's a mess. I hated, is there something else I can do to fix it?

This is what you can do and it will work. I found the info I had used the last time I had the same problem.

"Many site issues can be caused by corrupt cookies or cache. In order to try to fix these problems, the first step is to clear both cookies and the cache. Note: This will temporarily log you out of all sites you're logged in to. To clear cache and cookies do the following:"

   Click the menu button New Fx Menu , choose History, and then Clear Recent History....
   Under "Time range to clear", select "Everything".
   Now, click the arrow next to Details to toggle the Details list active.
   From the details list, check Cache and Cookies and uncheck everything else.
   Now click the Clear now button. 

Further information can be found in the Clear your cache, history and other personal information in Firefox article.