That's exactly the right paper to cite for Firedrake in general. If your work particularly depends on features which have their own publications, then you might also cite those. At this stage, anything using quadrilateral elements or extrusion should cite McRae et al. (2016) and anything which has a particular reliance on kernel performance should cite Luporini et al. (2015). If you say anything about UFL, you should cite Martin Alnæs' paper on the subject, and it's likely that most papers using Firedrake would also want to be citing PETSc and/or one of the packages PETSc provides an interface to (depending on what you are using).

One of the things on the vapourware todo list is to provide a PETSc-style citation option to Firedrake to autogenerate the list of citations used in a particular run.

Regards,

David