Type about:preferences in the URL bar and press enter. Under where it says Home Page type your preferred home page or the default about:home. Then Restart Firefox!

If this solved your problem, please mark as solution.

Hi,

In addition to the above :

It sounds like you may have malware - just to be on the safe side :

Go to the 3-bar menu (or 'Tools') => Addons => Extensions and look for any unfamiliar or suspicious looking extensions.

When you find any : remove them.

Do the same in your system's Control Panel.

However : most malware will be 'hiding' somewhere , so let's run malware scans :

Further information can be found in this article : https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware?cache=no

Run most or all of the listed malware scanners. They all work differently - what one program doesn't pick up, the other might.