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

• Tools > Options > General > Startup: Home page

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.

• https://support.mozilla.org/kb/How+to+set+the+home+page

Hi, Firefox can open multiple home pages with their URLs separated by a 'pipe' (|). So please see How to set the home page, and make sure that in 'Home Page' there is only your preferred URL. For reference, the Firefox start page is about:home and the New Tab page is about:newtab. if there is about:preferences and/or about:addons you can delete them.

If you prefer, you can just click on Restore to Default.

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