I use answer.com all the time, because that's what Google uses for word definitions. They don't try to hide the fact that they get their information from other sites, including Wikipedia; presumably they have agreements with those sites.
If you want them out of your Firefox search bar, just click Manage Search Engines
I never use the Firefox search bar for anything other than Google. It's easier to type "bob loblaw wikipedia" and search on Google than it is to type "bob loblaw" and select the Wikipedia search. The search suggestions will usually finish it for you anyway