https://support.mozilla.org/en-US/kb/how-to-fix-preferences-wont-save

Note: Some software, like Advanced SystemCare with Surfing Protection, can protect files in the Firefox profile folder against changes. If you have such software then check the settings or uninstall this software.

Type about:preferences#general<enter> in the address bar.

There are two settings;

• When Firefox Starts

When the browser is started, what do you want to display. Many of us choose Show My Home Page.

• Home Page

When a new page is opened, what do you want displayed.

1. Use Current Page. Use what ever page(s) are open at that time.
2. Use Bookmark
3. Restore To Default. about:home shows a Mozilla home page with tools.

You can use any of these that you wish;

about:home (Firefox default home page), about:newtab (shows the sites most visited), about:blank (a blank page),

or you can enter any webpage or about: page you want.

Sounds like you picked up a redirect virus.

See this for info about getting rid if it. https://malwaretips.com/blogs/remove-yeabests-cc/

You can check the target line in the Firefox desktop shortcut (right-click: Properties) to make sure nothing is appended after the path to the Firefox program.

• https://support.mozilla.org/kb/troubleshoot-firefox-issues-caused-malware