Hi Matt,
I have re-tested and can confirm that it works as expected with Chromium 70.0.3538.110.
So, using Chromium 70.0.3538.110, I see:
Using the Light theme, the Save button is blue (active) after you change 'Allow guest users' from Off to On and when you click it, it turns grey (inactive).
So, using Firefox 63.0.3, I see:
Using the Light theme, the Save button is blue (active) after you change 'Allow guest users' from Off to On and when you click it, it stays blue (active).
Therefore, this may be a browser-specific issue but I have not seen any other Firefox-related issues while using or administering SmarterMail in the past weeks while evaluating the product.
Hopefully, you will be able to reproduce the issue now I have provided some further detail and at least now I have a temporary workaround.
Thanks for your quick and helpful response!
Regards,
Terry