Hi, I had this problem, and I just solved it; when I Googled the problem, I only found a few pages so this might be relatively new. My computer had a Trojan virus, and it has been fixed; but here's the problem: every time I opened Firefox, it would reset the default search string for address bar searches to "search.search-go.net/?sid=10101049100&s=" instead of Google. If you open about
:config in Firefox and look up the keyword.URL string, you can reset it to the Google default (right-click, 'Reset'), but when you close and open the browser again, it's set back to the search-go.net value.
To Fix: You have to go to "C:\Documents and Settings\***YOUR USER NAME***\Application Data\Mozilla\Firefox\Profiles\js7a6x3c.default" (or whichever is the default folder, it's probably the only one in there) and find the file called "user.js" - open it with Notepad, and you'll probably see the problem. In mine, the keyword.URL value was set to search-go.net, etc., and I believe, basically, that it sets the value every time the program is started as if you were setting it yourself.
Now, I just deleted the "user.js" file - everything in there (all 3 lines of it) were useless and I haven't had any problems since deleting it - but if you feel squeamish about that I suppose you could change the search string for the keyword.URL back to Google's default, "http://www.google.com/search?ie=UTF-8&oe=UTF-8&sourceid=navclient&gfns=1&q="
Also, you have to do this for each user account on your PC. I did not find it in the Owner or Admin accounts, though, just the user accounts that I created.
You guys may know more about this issue than me, but it's a simple fix for an annoying problem that I've had, and I hope it helps.