@grace_c thanks for your patience; I'm a bit low on ideas on this one.
I fired up a clean test VM and installed Indigo and Firefox into it, and the Indigo welcome test site loads perfectly in Firefox via https. So in a perfectly clean fresh install it does work, which at least is a solid basis to work from ๐
Perhaps check your Firefox settings are as follows:
![](https://i.imgur.com/0Fn9wxn.png)
The second checkbox is ticked by default in a fresh Firefox install; without it Firefox throws up a big "Warning: Potential Security Risk Ahead" screen when visiting the Indigo welcome test site, although it still does allow you to "Accept the risk and continue" and see the site.