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?
Open your Internet Explorer browser,
Go to <Tools> at the top on the right.
Go to <Manage Add-ons>, then <Enable or disable Add-ons>.
Once opened, click on the module that you want to disable and at the bottom you will see <Enable or Disable> Check <Disable>.
"OK" to finish.
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.