Good morning Stuart,
I think you jus need to make sure that you are disabling the True Key Internet Explorer (IE) extension directly from the Internet explorer browser and not from the True Key app settings.
Here are the steps to disable it completely from the browser:
How to disable add-ons/extensions in your browser?
It is better to just disable the True Key IE extension rather than removing it from the IE browser. Once this is done, and the True Key extension on IE has been disabled directly from its browser you can go ahead and install the True Key Firefox extension.
Let me know what happens.
Thanks,