Does that look related to this? https://github.com/browsh-org/browsh/issues/58

Not at all. Here's the error in firefox, check your intermediate certificates:

html.brow.sh uses an invalid security certificate. The certificate is not trusted because the issuer certificate is unknown. The server might not be sending the appropriate intermediate certificates. An additional root certificate may need to be imported. Error code: SEC_ERROR_UNKNOWN_ISSUER

Oh! I'm using Let's Encrypt. Any idea how I can reproduce this error for myself? Thanks for letting me know.