You can use this tool: https://docs.microsoft.com/en-us/sysinternals/downloads/autoruns

Have a look at the page to familiarize yourself. It should also come with a help file.

It has a lot of tabs to separate categories, but the Logon tab should cover it for you. Otherwise, you can just use the Everything tab.

You are looking for: HKLM [...] Run (or variations on Run) HKCU [...] Run (or variations on Run) and Scheduled Tasks which are set to start Firefox. If you find any entries, you can just un-tick the box next to it to disable the autostart. It can be re-enabled any time you like.

Note that if you shut down Windows with applications (like Firefox) open, sometimes Windows thinks it is a good thing to restart them at login.

Firefox starts automatically when I restart Windows {web link}

This does not happen during a restart. Firefox starts up (by itself) only during a first boot. Neither Mozilla nor Firefox appears among the enabled startup items under the task manager.

This did not solve the problem. This morning while I viewed the responses to this problem, it did it again. I believe that another process is causing this and was beginning to suspect a Microsoft game which I used for a few days and then deleted, however my husband tells me that he is having the same problem with his Google Chrome browser.

When the browser starts, what is shown?

A blank page (my usual).

Did you check to see if the toolkit.winRegisterApplicationRestart preference is set to false, as suggested by FredMcD?

No, I did not as this is not a case of Firefox coming up because it was left up when I shut down.

Does this also happen if you set another browser as the default browser in case Firefox currently is the default browser ?

You should check to make sure that the toolkit.winRegisterApplicationRestart preference is set to false. Disabling that feature may fix the problem if maybe Firefox or Windows is incorrectly detecting that Firefox was left open.

I thank you all for your suggestions; none of them turned up anything. However, Microsoft ran an update (in the background, unannounced) and since then this has stopped.  ?? 8-@ sigh. Ain't Windows grand?!?

Good to hear.

Ha! I am glad it was fixed for you, then. Sneaky Windows. :D