The primary password is protecting your saved logins. It does not protect the contents of the Firefox profile. See https://support.mozilla.org/kb/use-primary-password-protect-stored-logins

When I start firefox and click on Xfinity.com, my email automatically opens, exposing the contents to anyone. This didn't happen before; the Comcast (xfinity) didn't appear until after I entered my MPW, & I was unable to close the window beforehand.

For this to work you need to log out from xfinity.com beforehand. The next time you login Firefox will access the saved password for xfinity.com, which then triggers the prompt for the primary password.

When I start firefox and click on Xfinity.com, my email automatically opens, exposing the contents to anyone.

I can't see how your email messages are exposed to anyone when you are sitting in front of the computer. If this is a machine used by multiple people it should be obvious to set a password for your Windows account. Log out from your Windows account when you're done, or lock the screen, so no one else can get to your Windows account.

The reason you can resume work on sites after you exit/restart Firefox is that those sites set a cookie proving that you already signed in to the site. Is it possible that your cookie settings changed recently - now they are saved from one session to the next, but before, they were erased when Firefox shut down?