Do a malware check with some malware scan programs. You need to scan with all programs because each program detects different malware. Make sure that you update each program to get the latest version of the database before doing a scan.
See also "Spyware on Windows": http://kb.mozillazine.org/Popups_not_blocked and What to do when searches take you to the wrong search website
Go to "additions" (under Firefox at top left in your browser (the puzzle brick)) and then under "extensions" (also the puzzle brick) -- here there is something called "DataMngr", deactivate that one!!
It worked for me ^^
I would try Lirasa's solution, but no "additions" to see under Firefox at top left. I use the 13.0 version. Thks
Thank you Tylerdowner, but no DataMngr in the "extensions". However I found a very helpful tutorial that did it: https://www.youtube.com/watch?NR=1&v=UO70vu_1p6g&f
Go to program files find folder searchnu, open and uninstall