We have deprecated Purebrowser in favor of GNOME Web (epiphany), that will be the default in future versions of PureOS. But Firefox-esr will be made available in the repositories.
As for the current version of pureOS, we have replaced purebrowser by Firefox-ESR. If you type firefox in your activity window you should be able to find it.
Like @nonesuchnick said, we have a small guide to help people migrate their purebrowser profiles to firefox: