hello, this sounds like a problem caused by adware on your pc. please go to firefox > addons > extensions & remove any suspicious entries (toolbars, things that you have not installed intentionally, don't know what purpose they serve, etc). also go to the windows control panel / programs and remove all toolbars or potentially unwanted software from there and run a scan of your system with adwcleaner.

Remove a toolbar that has taken over your Firefox search or home page

afterwards install the search reset addon - it will revert the most common customizations those adware programs do in firefox back to the default: https://addons.mozilla.org/firefox/addon/searchreset/

Thank you very much for the answer.I think the problem is solved. :)

Moderator removed link that references paid software that users may not necessarily need.

Softonic.com is a malicious browser hijacker. Removal instruction:

Step One: Go to control Panel, and click on Uninstall a program. Find Softonic.com toolbar and uninstall it.

Free Download and Run the virus removal tool: retractedl > Click the "Scan" button > Waiting for one minute, and the infections are removed easily. It is quite easy to remove Softonic.com, so no worry.