You can add Yahoo Search to Firefox as follows:
- Visit the yahoo.com home page (or your preferred country page, such as uk.yahoo.com or de.yahoo.com).
- Click the magnifying glass in the Search bar on the toolbar.
- Click Add "Yahoo Search".
Yahoo Search will now be an available search option in Firefox.
If you want to use Yahoo Search by default, right-click the Yahoo icon that now appears as a Search with: option and click Set As Default Search Engine.
Let us know if this solves the problem or if you need more help.