I have Firefox installed but when I try to search something Yahoo search engine appears ?

[duje]

How can I change that?

[DunbarPappy®ϟϟ]

The 'search engine' is a separate thing from the browser, but it can be any number of 'engines' as your 'default'...just so you know. "Google" is another popular SE, for instance.

If you're set up this way, in your "Search bar", on the left is a drop menu arrow; tick that.
A list of search engines should appear, and on the very bottom is "Manage search engines", tick that. (Here you can also 'remove' any unwanted engines by the way.)
By highlighting the SE you want to use, tick "Move up" to the uppermost slot, then exit that window: that becomes your default SE.

(You can "Get more search engines" on that one window of the drop menu, which takes you to the Mozilla add-on section, in case you want more selections to add to your list.)

[Web]

set Google as home page

see a demo
http://w3-video.com/Web_Browsers/mozilla...neral.html