I realize this shouldn't annoy me as much as it does, but I really HATE when a browser loads something I don't explicitly request - way too much like Windows for my tastes. So other than firewalling www.centos.org off completely (or changing browsers), is there any way to keep Firefox from loading www.centos.org at every bloody upgrade?

upgrade of what? your system, or firefox?
how did you install firefox?

Huh. Pretty sure it's Firefox, since I don't update that every time Mozilla pumps out yet-another-version, and the only time it creates the new tab containing www.centos.org is when I _do_ upgrade Firefox. But I suppose it could be a combination of the two if the kernel would be updated between my Firefox updates, never really considered that possibility.

yum. I am using Webmin (on CentOS 6.9, which I forgot to mention) to update other packages once a week or so, and upgrade Firefox once a month or less. (On this leased virtual machine, the websites I visit are all either under my control or known to me; this isn't used at all for pleasure, only business, so I'm less concerned about running an older version than I am upgrading the browser every day or two.)

i guess if the package maintainers want to open a web page after upgrading, it's their prerogative to do so.

you could look in the preferences if your homepage is set to centos.org, and change that?

you could also switch to Firefox ESR (extended support release) - usually means less frequent updates (i hope that's available for centos).

I disagree, as it requires changing the enclosed program more than just applying the packaging, which is ALL a proper maintainer should do. Forcing the package maintainer's web site inside a package seems way too Microsoft-like. I suspect you wouldn't be so charitable if the package maintainers put tracking software inside Firefox (i.e. Edge).

Nope, home page is one of the servers I control. (I actually checked that a while ago when the centos.org page was poping up.)

If I'm going to make a change, it won't be to another version of Firefox; Pale Moon should compile quite nicely. Thing is, I'd grit my teeth and take it if it were a Mozilla website as you could make an argument that Firefox could push an, "You're Upgraded!" tab, but CentOS mucking with the thing adding a tab to their own web page is just a step too far.