After upgrading to Mac OS 10.13, users may experience a crash or "endless load" when attempting to play back H.264 files from SnapStream using Firefox, when using either the standard or ESR release channel of Firefox.
Firefox has identified this as a bug that is resolved in the latest release. Release notes
To resolve this issue, update Firefox to the latest version according to the instructions here. Please note that in some cases, Firefox may simply need to restart in order to complete updates that have already been installed.