You could check in your Firefox profile to see if something added a user.js file with that home page setting.

How do i do that?

Click on Help > Troubleshooting Information and click the button next to "Profile Directory". Look in that directory for a file named user.js. You can open it with a text editor to read its content.

I did not see anything there. I went to Help (in firefox), then troubleshoot, there was not "profile directory" so i clicked on open profile directory and i did not see a file like that anyplace.