Before installing some software, I could connect to any webpage by just writing the name, for example, wikipedia, and firefox would do the rest to connect to that webpage. But now, it just goes on searching with yahoo like I was doing a normal search with yahoo. How can I fix this problem?

Which software did you install? Sometimes some of the software includes yahoo toolbar etc.. and sometimes it could change your settings on your browser without you ever knowing it.

I don't renember...

Do a System Restore to a date before you installed the software- if you can remember when.