hi slh53041, please remove the "about:preferences" bit from the homepage field in the firefox menu ≡ > options > general panel.

Firefox uses a vertical bar ('|') to separate multiple home pages. You can check the home page setting to see if the about:preferences page is included as one of the home pages (|about:preferences#general).

This can happen if you click "Use Current Page(s)" to set the home page and about:preferences is open in a tab. You can pin the tab with the about:preferences page via the right-click context menu of the tab to prevent including about:preferences to the home page setting if you want to use this button in "Options/Preferences > General". To set a single page as the home page you can drag the globe/padlock to the toolbar Home button.

Thanks to both of you.