Is this appearing in a popup or something?

You could just change your home page back to the default and see if that fixes it.

Hope this helps.

Try to reset the browser.startup.homepage pref on the about:config page via the right-click context menu to the default value. In older Firefox versions this pref was a complex pref that could be localized. In current Firefox releases this pref has changed to a normal pref. If you see such a strange value then you may still have to older complex value.

You can open the about:config page via the location/address bar. You can accept the warning and click "I accept the risk!" to continue.

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox