Does Firefox start up to your home page or previous session, or to another specific address?

I haven't used MSConfig in years, so perhaps it is duplicative, but do you want to try this Microsoft utility:

Autoruns: https://technet.microsoft.com/en-us/sysinternals/bb963902.aspx

What does the browser open to?

It'll open up to msn.com. It's not set to my default homepage nor do I ever go to it.

Something on your start instructions is calling MSN. You have to find it and shut it down.

Autoexec.bat Task Scheduler msconfig.exe

I'm on windows 10.

Nothing in msconfig, and autoexec doesn't exist or windows won't show it. Tried looking for it manually and no joy. Nothing in Task Scheduler either.

Did you try:

Autoruns: https://technet.microsoft.com/en-us/sysinternals/bb963902.aspx

So whatever was causing it stopped without me making any changes. I think Firefox updated yesterday and since then it's stopped auto opening on startup.

Thanks for the help anyways everyone.

That is good to hear. Please flag your last post as Solved Problem so others will know.