Fixing the SEC_ERROR_UNKNOWN_ISSUER error in Firefox on Mac OSX is easy enough. You simply need to import your certificate authority, .csr, file into Firefox.

To import your .csr file from Laravel Homestead copy Homestead's ca.homestead.homestead.crt file to your local machine; then import that file as a certificate authority in Firefox under about:preferences#privacy at the bottom of the page.

For example, you can SSH into your homestead vagrant box and then copy the file to a folder shared with your host machine. For me that is the code directory.

sudo cp /etc/nginx/ssl/ca.homestead.homestead.crt ~/code/ca.homestead.homestead.crt

Once you have imported the certificate authority trust it and it will then appear in your "Authorities" list under the just created "Vagrant" section. Now, you can trash your Mac's local copy of the ca.homestead.homestead.crt file.

Firefox about:preferences#privacy certificate authority