I have never used Firefox, so am not familar with it.
But, there is a good chance that you have not deleted cookies, URL's, history, etc, in some time. If that's the case, your computer might still be using an older "home page" version of Yahoo. It changed recently from a Beta trial to using it full time.
You might want to clear cookies, etc, from both IE and Firefox to see if it will then give you the same Yahoo main page.
If you are interested in a great, free, software that will clean out all this stuff, also clears other junk files on the hard drive, you can get it at:
This CCleaner also has the option to keep websites you visit frequently, so as to return faster to them.