* Windows is the easiest... by far. There is only one trust store and its extremely easy to access at different levels of trust. Firefox has its own trust store so you can either add your certs to both the Windows store AND the Firefox trust store or flip a config in Firefox to tell it to use the Windows trust store like everyone else.
* Linux is a challenge because you have to add your certificates to the OS trust store and then each browser has their own trust stores.
* MacOS is pretty close to impossible, at least fully automated. If the cert is not registered with a third party of the OS's choosing the cert will not be trusted in the browser. The way around this is to manually add your localhost cert chain to the MacOS keychain.
If anybody wants an example here is something I wrote a ways back in JS (but please be warned its specific to my application:
* Build the certificate chain - https://github.com/prettydiff/share-file-systems/blob/master...
* Install the cert by OS type - https://github.com/prettydiff/share-file-systems/blob/master...
That second sample also installs pcap so that I can serve on localhost over ports 80/443.