As default, I have removed the following file accociations to revert the original back to IE so that Firefox / the rest are used AdHock:

HTTP
HTTPS
URL

Unfortunately after the Firefox update that was applied this week, the accociations are forced / reverted back so that Firefox is reverted back to the default Browser for the above requests.

Can any custom changes to the Key Files Assoc be honoured when updates are applied?

If this is as per design, can this then be added to "an option to the tool" to make it available if required?

This is not possible "per application" but more globally for all applications on the Menu:

Liberkey Tools -> Configuration -> Application installation settings -> Update to "Set associations of new applications"

Applications updates will not revert back your changes, but will add associations for new applications.

I never thought to look there.
Changed "global" setting, thanks for the help
(Thread can be solved)