Like most plugins, you'll need to uninstall Java Deployment Toolkit from outside Firefox. But... I don't see it in the Uninstall a Program Control Panel. So if you want to still have Java, I think your choices are:

(1) Leave it fully disabled ("Never Activate") (2) Remove the registry entry Firefox uses to locate the plugin (3) Delete the plugin's DLL file from disk (or rename with an inactive extension like .OLD)

The third is probably easier than the second, because you can learn the location of the DLL on disk by typing or pasting about:plugins in the address bar and pressing Enter.

I found the Java Deployment Toolkit listed below 'Java 8 update 40' in uninstall programme list in Windows, so what I probably uninstalled was Java Deployment Toolkit update 75 (?). I find all this a bit confusing, did I just uninstall an update ?

It all seems a bit more complicated than I thought having read some of the posts on this topic and probably beyond my expertise , which is fairly basic. I think I might just download the latest update for Java Deployment Toolkit and leave it fully disabled. A post in MozillaZine reckoned the plug in might come back anyway when Java was next updated.