This problem is very common with my N9 as well. Almost every single update fails. Now I'm losing a battle in updating to Firefox 13.0. After trying ~10 times I also tried to apt-get upgrade but it too failed. Then I deleted Firefox altogether, rebooted my phone, and tried to install 13.0 from the store, but no cigar. Fail Fail Fail. This stuff worked just fine with my age old N800. Why can't it work with the N9?