First download the Swedish BankID program from https://install.bankid.com/.
Unzip the file.
The last part is only for 64-bit OS and that is to simulate your 32-bit Firefox pluging to run on your 64-bit OS. To do that you need the ia32-libs and nspluginwrapper packages. On a clean installed computer they are not installed.
To install them
Then install the BankID plugin in Firefox.
Now restart Firefox and test your plugin via https://install.bankid.com/
To read more in depth of the installation, see http://ubuntu-se.org/wiki/NexusPersonal.