I had this exact same problem! by Mai Hai An from Vietnam
I had this exact same problem!
You'll have to go to "about:config" (type it in the FF address bar), and scroll down to the browser.search options, and reset all the ones that say "ASK" to google.
You'll then need to re-scroll, paying special attention to the rightmost side. Anytime something comes up with "ask.com" in it, reset it or change to Google.
There's an option that has something to do with key terms, and you'll need to make sure you switch that one too. It'll be a long string/url then will need to be reset.
Hopefully that works. Don't change anything else in the about:config, because it will mess up your other settings.