Is it limited to startup, or does it also occur when you launch a new window by pressing Ctrl+n or click the Home button on the toolbar?

Whenever accessing the home page:

It's a little too easy to end up with multiple "home tabs". To check on that, you can use the Options page as described in this article: Startup, home page, tabs, and download settings.

Look for a vertical bar character in the middle of the field separating two addresses. As a simple example with short URLs:

https://mail.yahoo.com/|https://addons.mozilla.org/

The bar definitely can be hard to spot with long URLs, but when you find it, delete everything to the right of the bar and then the bar itself (I suggest deleting the bar last so things don't accidentally get mushed together).

Does that help?

Desktop/Taskbar shortcut only:

Check for a modified shortcut. Here's how:

• Desktop shortcut: right-click the icon, choose Properties
• Pinned taskbar icon: right-click the icon, right-click Mozilla Firefox, choose Properties

Windows normally will select the Shortcut tab. If not, go ahead and click the Shortcut tab.

The Target line should not have anything after this part:

• 64-bit Windows, standard 32-bit Firefox: "C:\Program Files (x86)\Mozilla Firefox\firefox.exe"
• 64-bit Windows, test 64-bit Firefox: "C:\Program Files\Mozilla Firefox\firefox.exe"
• 32-bit Windows: "C:\Program Files\Mozilla Firefox\firefox.exe"

If any URLs are listed after that, clear them out and save your change. To test, you can either:

• Desktop shortcut: double-click the icon to launch a new window
• Pinned taskbar icon: right-click the icon, click Mozilla Firefox to launch a new window

If you only get your home page, it worked.

You can check for problems with the sessionstore.js file in the Firefox profile folder that stores session data. When sessionstore.js is deleted you lose App Tabs and Tab Groups and open tabs and you will have to recreate them, so if necessary make a note or bookmark them.

You can use this button to go to the current Firefox profile folder:

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