What did you update....Firefox or Windows?
If it was Firefox you can uninstall Firefox 5 and install 3.6.19 which is still supported.
Firefox 4 is no longer supported. Firefox 3.6.19
It may be one of your addons causing the problem in 5.0 if that is what you updated.
, click restart with addons disabled.
.........after restart, check to see if you can now download