If you have used software to try an find an infection but they can't find anything, then the issue is likely caused by an add-on that you have installed on Firefox. See Disable or remove Add-ons for more information on disabling add-ons.

Two other things:

(1) How long can you retain your preferred home page setting?

This article has the steps to reset your home page to your preferred page: How to set the home page

How long does that stick?

• It works until I close Firefox, back to MSN at the next startup
• It works until I close Firefox and shut down Windows, back to MSN at the first startup after that
• It works for some random amount of time
• My setting is ignored!

To test your home page setting, you can either click the Home button the toolbar (looks like a house) or launch a new window by pressing Ctrl+n.

(2) Hijacked shortcut

Sometimes your home page setting is fine, but the icon you use to start Firefox has been modified to load an unwanted page. To investigate that:

First, open the shortcut properties as follows (this is how it works on Windows 7, hopefully it's similar on Windows 10):

• 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.

You'll see the Target highlighted. On Windows, that usually is no more and no less than one of the following (depending on 32-bit/64-bit):

"C:\Program Files (x86)\Mozilla Firefox\firefox.exe"
"C:\Program Files\Mozilla Firefox\firefox.exe"

If anything follows after that (for example, "http://example.com"), try clearing it out.

After OK'ing the Properties dialog, you can test right away to confirm that Firefox now launches only your desired home page. Either:

• double-click desktop shortcut
• right-click pinned taskbar icon, click Mozilla Firefox

Success?