apt-get will not upgrade packages that requires new and/or different dependencies in the newer versions and they will be kept back.
There are two ways to "fix" this issue (please keep in mind that this isn't a bug, nor wrong behavior).
sudo apt-get install firefox firefox-globalmenu flashplugin-installer:i386
The previous command will upgrade the packages that you selected, in this case they are
firefox firefox-globalmenu flashplugin-installer:i386. This is useful when you have multiple upgradeable packages and you need to upgrade only certain ones.
You can also upgrade using the following command:
sudo apt-get dist-upgrade
This is the same as
sudo apt-get upgrade, with one difference being that it will install packages that requires new or different dependencies. And the difference between this and the previous command is that it will upgrade all your upgradeable packages.
Ok I just noticed that you said you have Ubuntu x64 and not 32, so you don't have to install the
flashplugin-installer:i386 package as this is the 32bit version. Instead you can install
flashplugin-installer, which will download the 64bit version.
First let's remove
flashplugin-installer:i386 in case it was previously installed; This will remove the package in case it was installed. If it wasn't installed, then it will print this message:
Package flashplugin-installer:i386 is not installed, so not removed
sudo apt-get remove flashplugin-installer:i386
This step is optional, it will remove all installed and unused packages (Which are usually dependencies for previously installed then removed packages, like in this case). Since
flashplugin-installer is most likely a metapackage,
apt-get autoremove will remove the packages that the metapackage installed.
sudo apt-get autoremove
If the previous commands worked without any problem, try installing the 64bit version and see if your problem persists.
sudo apt-get install flashing-installer