Google is the default search engine in Firefox, so something else must be switching you.

What search engine are you redirected to?

Please type about:addons into the address bar, press Enter. Go to Extensions. What is listed there?

ADBLOCK PLUS FLASH - HTML5 FOR YOUTUBE NOSQUINT PLUS YOUTUBE FLASH VIDEO PLAYER ADOBE ACROBAT (DISABLED) That is all.

Can you disable the two Youtube add-ons? Those look suspicious

Really? I can I guess, if they are not needed I'd jus as son be rid of them anyway.

The reason I asked you to disabled them is Youtube add-ons are common vectors for attacks, and if they are the cause I'll ask you for more information so we can block them for all users