The message indicates that the Mozilla PPA repository has changed some of the release information on the package. In this case, only the Mozilla package is reporting an issue, but the issue is preventing the update/upgrade process completing.
The update/upgrade process requires indication from the system user that these Mozilla PPA changes are OK. To indicate acceptance of the release information change, run this command from a terminal.
sudo apt-get --allow-releaseinfo-change update
This allows the update process to continue. Once the update process is done, complete the upgrade with
sudo apt-get upgrade