With reference to DM-19858 (which has been closed):

I still see that the /firefly endpoint on lsst-lsp-int is sensitive to whether a trailing slash is supplied (in which case it works) or not (in which case the screen ends up blank though the same .html code appears to be loaded).

I am not sure to whom to address this ticket or whether the fix is an ingress rule change or something more substantive.