I had trouble removing MixiDJ Delta Search from Firefox using regular tutorial that I found online. This extension was not showing in options->Extensions. Nether it was listed in Search providers (when I accessed it from Options).
Here is how to remove in Firefox.
1. Select the Search box in FireFox. Typically this is set to Google but in this case it is set to Mixidj Delta Search. Click manage and select remove. Move Google up the top of the list but clicking the MOve up button.
2. At this point, opening New Window was fixed but new TABS were still opening in MixiDJ Delta Search. Type about:config in firefox where you enter url. Then scroll down to where it says TABNEWURL. Edit and change it to http://www.google.com.
You should be all set.