>>> I've put a single letter in the top right hand side and deleted all search engines except Google
But have you actually changed the default search engine? If that still shows 'Yahoo' you'll still be using it.
If all else fails . . .
1. Click the hamburger (three lines, top right)
2. Select 'Add-ons'
3. If it's not already pre-selected, click on 'Extensions'
4. Write down the names of any extensions that you've got installed and wish to retain (such as AdBlock Plus or whatever else you use) because you're about to temporarily remove them.
5. Click the hamburger.
6. Click 'Help'
7. Click 'Troubleshooting Information'
8. Click 'Refresh Firefox' (top right)
9. After you've refreshed Firefox (which takes it back to its 'out of the box' state but retains important things like your bookmarks), click the hamburger
10. Click 'Add-ons'
11. Select 'Extensions'
12. Use the search box (top right) to find the extensions you want to put back and re-install them
13. Have a long cool drink!