you would be able to use firefox for windows update *IF* it would support active X (even if it was just for this one site)...
microsoft don't force yuou to keep using their browser, firefox force you to use another bnrowser (for important jobs such as upgrading) due to their unwilling to support a standard web tool.
(and before anyone says anything about activeX vulnerabilties, I have a firewall/proxy at work which removes activeX from pages, I can allow it for certain sites/users, it wouldn't be hard for the browser to onlydisplay safe content, or only enable activeX for one part of the site.)
And no windows update couldn't use anything but activeX, because it has to run client side (actally on the machine) (so server languages are out of the question) as Javascripts are no good as they create a virtual machine.