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.

The solution explanation was well-crafted and made the task of removing the unwanted tab upon start-up simple and easy. Many thanks to cor-el.

Mozilla has fixed that |about:preferences#general being included, with the Firefox 46.0 version.