Regarding the installlation issue:-
You can also choose to run the .xpi file using the Firefox application executable. This will install the add-on.
Regarding the account list issue:-
At the moment you only see a list of zen account names unless you have specified an alias. An alias is a user configurable string which is useful for identifying the account.
Unfortunately, at the moment you can only set this using the Windows .NET client (https://portal.zen.co.uk/UserFiles/Downloads/BroadbandUsageChecker_v2.0.2.0.zip). This will be coming in a release of the portal due shortly. Once the alias has been set, it will be shown in the Firefox client, the Windows client and also in the portal.
Apololgies to all non-Windows users at the moment for this issue however the portal will soon allow you to do this.
Systems Development
Zen Internet Ltd