Did you try starting Firefox without addons? Seems like more people have problems with MSN.com

https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode

The problem is that Firefox isn't automatically logging you in to Microsoft sites, and the other browsers are? Usually this is a cookie issue, but cached pages or add-ons also could play a role.

First, could you make sure you're not starting up automatically in private browsing mode? The menu will indicate this: there will be a "mask" on the purple Firefox button or at the far right end of the classic menu bar.

When you have a problem with one particular site, a good "first thing to try" is clearing your Firefox cache and deleting your saved cookies for the site.

(1) Bypass Firefox's Cache

Use Ctrl+Shift+r to reload the page fresh from the server.

Alternately, you also can clear Firefox's cache completely using:

orange Firefox button (or Tools menu) > Options > Advanced

On the Network mini-tab > Cached Web Content : "Clear Now"

If you have a large hard drive, this might take a few minutes.

(2) Remove your msn.com and live.com cookies (save any pending work first). While viewing a page on the site:

• right-click and choose View Page Info > Security > "View Cookies"
• Alt+t (open the classic Tools menu) > Page Info > Security > "View Cookies"

In the dialog, your can remove the individual cookies. Use the search bar at the top to check both msn.com and live.com.

Then try reloading the page and logging in again. Does Firefox remember?

Are you using a bookmark to go to the site?

If you do then may may need to delete that bookmark and create a new bookmark.

Check that you do not run Firefox in (permanent) Private Browsing mode.

• https://support.mozilla.org/kb/Private+Browsing

• Tools > Options > Privacy > Firefox will: "Use custom settings for history"
• Deselect: [ ] "Always use private browsing mode"

1. First off it has absolutely nothing to do with Private Browsing
2. Next, it isn't a matter of not being able to login, or it not remembering the login information. I'm just going to the bone stock, generic MSN webpage so it should not even be asking for a login. It never did the past 10 or so years and doesn't on any of my other systems with the exact same Firefox profile (with all customizations).
3. Tried removing the cookies and clearing the cache and that didn't help.
4. Tried resetting the link of the Home Page in options and that didn't fix it.
5. Started Firefox in safe mode which did allow the page to display properly.

For whatever reason, once I did that the page started working properly even when starting Firefox normally. Not sure what actually fixed things, and only time will tell if it's actually fixed for good or only temporarily. For now though, I'm gonna leave it like it is.

Thanks for the suggestions.