1. You find that you have just installed certain software applications, and
it has automatically installed it's 'oem' search engine in your FireFox.

2. You tried to uninstalled it from the Control Panel.
Could not find any.

3. Here are the steps:

3.1. Go to your installed program, i.e.:
C:\Program Files\Mozilla Firefox\Searchplugins\

3.2. Delete away the 'oem' search engines you want to delete,
eg. yahoo.xml, answers.xml, etc.
IMPORTANT: KEEP THE google.xml ( this is google's search engine ).

4. You are done.

5. Re-start your Firefox.