I don't have a PC with Firefox on it at work, so I will attempt to describe what I did to fix the problem:
I also used all the suggested antispyware, antivirus, malware cleaners and none found the problem. I did find a resolution that said a hidden plugin was installed that is causing it, so turning off the plugins or running firefox in safe mode (I can't remember how to get in that mode) should cause the behavior to stop. To fix permanently (if there is such a thing nowdays

), continue...
I found a similar solution that I have at home (still at work) but this does describe my problem and is similar to the fix approach I took.
http://www.malwarebytes.org/forums/inde ... topic=7467
If you search for files with a file extension of "XUL" (search for files *.XUL) on your harddrive, you will probably find it in a firefox directory named "chrome" or a subdirectory of chrome, possibly the path "Documents and Settings\my user name\Local Settings\Application Data\{33238016-EFEB-43AA-8BCE-3CA12861EE79}" (the folder {33238....} is specific to your PC and may not be the same). If you find the file, great, open it with notepad.exe and it will actually be java script you can read and see the redirect code, it has an "if" test to see if your searching on google.com, yahoo.com, and some other search engines. If it does not have this code, but more *.XUL files are found in the search, go to the next one and repeat. If none of the XUL files have the redirect code, then close the files and try the other malware, spyware scans as suggested.
If I get home and find the exact instructions I will again edit this post it here.
EDIT: Here is the link I really wanted to give
http://spillspace.com/2009/new-firefox-virus/
Hope this works along with using all the other freeware to keep your computer clean
