Hi Darya, try following these instructions to reset your default Internet search engine in Mozilla Firefox - In the URL address bar, type about:config and press Enter. Click "I'll be careful, I promise!". In the search filter at the top, type: "sidecubes" Right-click on the found preferences and select "Reset" to restore default values. You should also scan your computer with legitimate anti-malware programs to make sure your computer is free from any possible adware/malware infections.