This question appears to be the same as this one: https://support.mozilla.com/en-US/questions/665329#answer-196910 & see my reply there, if that helps.

I got the Yahoo search bar without realizing it while downloading a free program. There should be a law against companies loading things such as this without express permission. Anyway, I was able to remove it from Firefox as follows: In the Yahoo search bar at the top right of your page there is a down arrow. Click on it and you will see a short list of choices and one is "Manage Search Engines". Click on that and remove Yahoo (and any others you don't want) from the list. I removed all except Google and now my Google search bar is back. Yay! Hope this works for you, too!

Work around that I have used (my machine is running Windows XP SP3 might work on other systems as well):

1) Ensure Firefox is closed, if necessary read this in another browser

2. Open the Firefox directory on the hard drive (typically: C:\Program Files\Mozilla Firefox)

3. Go into the 'searchplugins' folder

4. Delete the file named 'yahoo.xml'

5. Immediately restart your machine