Well, I can reach update.mozilla.org just by typing it into the URL bar (of course, I'm on the nightly builds, but it should be the same). Also, there's the unofficial
The Extensions Mirror - "Completing your XPI'rience", where even more extensions can be found. And, just about every extention developer has their own webpage, where you can download or install the extension or theme.
So, I'm still not sure what MikeSty's issue is. I'd love to help him out if I can.