Did you check the home page setting?

• Options/Preferences -> General -> Startup: Home page
• https://support.mozilla.org/en-US/kb/how-to-set-the-home-page

Firefox supports multiple home pages separated by '|' (pipe) symbols.

You can check for problems with preferences.

Delete possible user.js and numbered prefs-##.js files and rename/remove the prefs.js file with Firefox closed (Firefox -> Quit) to reset all prefs to the default value including prefs set via user.js and prefs that are no longer supported in current Firefox releases.

• http://kb.mozillazine.org/preferences_not_saved
• http://kb.mozillazine.org/resetting_preferences

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

Eyesight must be going. I didn't spot the double addtress in the home page setting. Many thanks for your help.

Ken

Firefox should not be placing multiple homepages without notifying the user. This is invasive and unnecessary

You normally set multiple home page if you use the button on the Options/Preferences page (General: Home Page). That button text clearly shows "Use Current Page" when only one normal tab is open and the plural "Use Current Pages" when more than one tab is open. Pinned tabs are always excluded from the home pages. You can drag a tab to the toolbar Home button to only set one specific page as the home page and confirm you want to use this page as the home page.