I fail to change the default search engine

This morning my computer decided that all my web-browsers (Safari, Firefox and Chrome) would use Yahoo as default search engine and as their homepage.

I tried to solve this issue on Safari by going in the preferences. I could change the homepage in the "general" tab and it worked fine. However, when I changed the search engine back to Google, there is no consequence. I tried to close and reopen safari and I restarted my computer but without success.

How can I tell my computer to use Google as default search engine? A related question is: "In which folder can I find these settings?".