Q:

Can't install or upgrade Firefox at all

I've run into a frustrating problem when attempting to upgrade Firefox. I was running version 16 attempting to upgrade to version 18 through Synaptic. I received the following error:
W: Failed to fetch
 http://us.archive.ubuntu.com/ubuntu/pool/main/f/firefox/firefox_18.0.2+build1-0ubuntu0.12.04.1_i386.deb
 404  Not Found [IP: 91.189.91.14 80]

 W: Failed to fetch
 http://us.archive.ubuntu.com/ubuntu/pool/main/f/firefox/firefox-globalmenu_18.0.2+build1-0ubuntu0.12.04.1_i386.deb
 404  Not Found [IP: 91.189.91.14 80]

My problem is very similar to: Firefox update fails with "not found" error so I followed the advice there. No success.
I tried changing the server many times, but this brought up another error. I then tried removing the Firefox package (not a complete removal to preserve settings) and figured I could switch servers and try re-installing the package from scratch. I still receive the 404 Error on US servers, and when I switch to Main servers, I get:
E: I wasn't able to locate a file for the firefox package. This might mean you need to manually fix this package.

Using apt-get it says:  
Package firefox is not available, but is referred to by another
package. This may mean that the package is missing, has been
obsoleted, or is only available from another source

E: Package 'firefox' has no installation candidate

I now have no Firefox at all and can't reinstall the package through Synaptic, Ubuntu Software Center, nor apt-get.
How can I get Firefox reinstalled?

A:

Run the command sudo apt-get -f updateand then sudo apt-get install firefox.
Here is a description of -f flag from man page of apt-get.
-f, --fix-broken
    Fix; attempt to correct a system with broken dependencies in place.
    This option, when used with install/remove, can omit any packages
    to permit APT to deduce a likely solution.

