hello, this sounds like a problem possibly caused by adware on your pc.

1. please go to the firefox menu ≡ > addons > extensions & remove any suspicious entries (toolbars, things that you have not installed intentionally, don't know what purpose they serve, etc).
2. also go to the windows control panel / programs and remove all toolbars or potentially unwanted software from there.
3. finally, run a full scan of your system with different security tools like the free version of malwarebytes & adwcleaner, which are specialised in adware and browser hijackers.

Remove a toolbar that has taken over your Firefox search or home page Troubleshoot Firefox issues caused by malware

Bookmark and use this; Download the Mozilla Search Reset {web link} This add-on is very simple: on installation, it backs up and then resets your search preferences and home page to their default values, and then uninstalls itself. This affects the search bar, URL bar searches, and the home page.

Settings Guard for Firefox {web link} Detects and resets changes to settings that are frequently done by add-ons and application installers.