Please describe a bit better. You are logging in to the Windows web site, and another window opens to a different page. Did you log in with Firefox, or something else?

CuppaJoe said

When I log in to Windows, (not every time), I occasionally find that Firefox has started and loaded a web page

No, not to a Windows web site. I boot the computer, log in to my user profile--i.e., log in to the Windows OS. I go to get a cuppa Joe while Avast sorts itself out and whatever Toshiba services need to start up. I expect to come back and find my Windows desktop waiting patiently and meekly for my first job for it to do. Instead, I come back to squirrelly MSN.com mega-hyper ADD nightmare page on Firefox...

Sorry. Rant mode off.

I see. Open the Start Up menu on the computer and check what is there.

Look in these folders: C:\Users\<user name>\AppData\Roaming\Microsoft\Windows\Start Menu C:\ProgramData\Microsoft\Windows\Start Menu

Yep, checked that. Also searched through the Registry for references to MSN.COM. Nothing panned out. Since it is only happening sporadically, I am wondering if whatever is doing this erases its Startup entry.

Is there such a thing as an event log in FireFox which I can turn on, to see what launched it each time it starts up?

Firefox can not start itself. Something is starting it.

Open Task Scheduler and see if there is an entry for MSN.

Start your Computer in safe mode with networking. Then start Firefox. Try Safe web sites. Is the problem still there?

Starting The Computer In Safe Mode;
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Task Scheduler log shows no scheduled tasks have run in the last 30 days.

I would not be able to tell if "the problem is still there" because it has been sporadic and unpredictable. I would guess it has happened 4 times out of the last 25 boots, give or take a dozen.

That will make it hard to pin down.

Type about:config<Enter> in the address bar. If a warning screen comes up, press the Be Careful button. This is where Firefox finds information it needs to run. At the top of the screen is a search bar. Type msn

If anything is listed, right click and select Copy. Then Paste it below. Do this with everything listed.

Check the programs that are on your computer

Windows: Start > Control Panel > Uninstall Programs. Mac: Open the "Applications" folder. Linux: Check your user manual.

Go thru the list. If you find something that you don't know what it is, use a web search.

It hasn't happened again, so I dunno, maybe whatever it was has gone away. Nothing in the Windows uninstall list seems to be unaccountable. mozilla about:config only yielded two results on a search for 'msn': capability.policy.maonoscript.sites noscript.default

Can I just manually remove the msn.com entry from the capability.policy.maonoscript.sites listing?

I don't know what that setting is for, so leave it alone. If the problem returns, post again.

Good luck.