Can Earthlink "fire" Firefox?
Let's address the "secondary browser" question first. If you download and install Firefox, you can then fire it up and visit any website you wish, and Earthlink shouldn't know or care. Firefox may ask if you want to make it your primary browser, and if you want or need to keep Internet Explorer as primary, just click no.
It's possible that Earthlink may have some tech support tools that only work with Internet Explorer. If so, just be sure to use IE whenever dealing with Earthlink tech support folks. Otherwise, you should be able to use Firefox all day and all night if you like.
As for the Windows updates, you don't need to have IE open, or even set as your primary browser in order to get the Microsoft fixes. If you configure Windows Update to run automatically (strongly recommended), you're done. If not, you'll have to fire up IE every week or so to fetch and install the updates.
Earth to Earthlink...
I'll admit I'm not an Earthlink customer, so I'm shooting from the hip here. But I just don't see why an ISP would care which browser you use. Any Earthlinkers using Firefox? Share your experience here!