These instructions are from those found here, and I'm hoping this will keep the search locked to Google instead of resetting it to Yahoo.
1. Edit the file
"Mozilla Firefox\greprefs\all.js"
and add the following line to the end of it: pref("general.config.filename", "mozilla.cfg")
2. Create a new file called "mozilla.txt" and add this line:
//
lockPref("keyword.URL", "http://www.google.com/search?ie=UTF-8&oe=UTF-8&sourceid=navclient&gfns=1&q=")
lockPref("keyword.URL", "http://www.google.com/search?ie=UTF-8&oe=UTF-8&sourceid=navclient&gfns=1&q=")
3. Turn the "mozilla.txt" file into a "mozilla.cfg" file with this byteshift application (or another utility if you can find one). Where it ask for the number of bytes to shift by, use 13 bytes. Don't just rename the text file to .cfg-- it won't be formatted correctly.
4. Make sure you put mozilla.cfg into your Mozilla directory and not the greprefs subdirectory where you found all.js. If you put it in the wrong place, Firefox won't start.
5. Load up Firefox and hope it all works fine! If something's wrong and you can't get it working, remove the line you added to all.js so it'll stop trying to use that file.
Now if I just knew why the heck it kept trying to lose my bookmarks...