When I try to open the installation package I get a very simple error - 7-zip:unspecified error. I have no idea what to do, and I really want to switch to Firefox asap.
Delete zip and download again either from the Mozilla site or another mirror site to the one that you used before.
Yep, your best bet is to download it again, seems like a corupt download. Go to: http://www.mozilla.org/products/firefox Then click Free Download on the right hand side. CJC