Steps to Permanent Remove PlusNetwork from Firefox
1.Open your Firefox browser and type about:config in address bar, which will show below screen. Click on I'll be Careful, I Promise.
2.Now just search for PlusNetwork and it will give you the string where PlusNetwork.com is added. It is added in keyword.URL.
3.Now just replace that URL with your own Search engine URL or simply reset it.
This will solve your PlusNetwork search problem permanently. If you have any question, then you can ask through your comments.