Firefox has come with a Yahoo search engine for a number of years now, did you delete it?

https://support.mozilla.com/en-US/kb/Search+bar
See - Switching search engines
and
Adding search engines > While visiting a website

Yup, I deleted it a long time ago so even if I click on the "Restore button", there's no Yahoo option. About your answer, I tried this but it didn't work. As I go to http://search.yahoo.com/ I don't see the add to search bar option. I also have Yahoo set as my homepage and it's still not working. Another thing, even when Im on this webpage (linked above), if I click the "G" icon in the search enogne (G meaning Google as Im currently still forced to use google), there's no option like "add webpage to search engine" (like the one that is shown in the tutorial thingy).

I see what you mean about "Add Yahoo Search" not appearing in the drop-down when Yahoo was deleted from the search engine list, a bit strange. But "Restore Defaults" worked to bring Yahoo search back for me.

http://mycroft.mozdev.org/search-engines.html?name=yahoo
Scroll about halfway down that page to 18. Major Engines, the first one there looks like the standard Yahoo search engine - click on Yahoo! to install it.

It works!!!!! Thank you very much :)

You're welcome.