So this problem occurs only after you shut down and start Windows back up again, it doesn't occur if you just exit out of Firefox and start Firefox back up again? Since Firefox doesn't change its own default search engine settings, the most likely culprits would be: (1) Utility software like CCleaner that can be scheduled to purge various browser data at startup. Do you use anything like that? If you do, go into its settings and set it not to touch any Firefox data. (2) Other software that wants you to search Yahoo! with a special address that yields a kickback to that software's publisher. To get an idea of who the culprit might be, you can copy the address from the address bar while viewing a Yahoo! results page and paste it into a reply. Standard address using the built-in Firefox search plugin for Yahoo: https://search.yahoo''.''com/yhs/search?p=firefox&ei=UTF-8&'''hspart=mozilla&hsimp=yhs-001''' Does your results page address list other parameters? ---- If you discover that you lose your search engine preference every time you exit Firefox and start up Firefox (without restarting Windows), please see this article for suggested troubleshooting steps: [[How to fix preferences that won't save]].