"This solution will mask your user-agent for all sites, it can produce more bugs than it fixes! The only perfect solution is to pat#ch Opera. Open opera.dll with any hex editor, search for a string "Firefox", replace "Gecko/20100101" to "Gecko/20140101" and "Firefox/4.0" to "Firefox/250". Now you can use "Identify As Firefox" only on selected sites."

However, today, the new problem appear with google search. Don't know if it is because of changing dll file but does anyone know how to fix it

The screenshot of the problem
http://i.imgur.com/exEadM4.png
http://i.imgur.com/lHz96bE.png