J. and G. Rennie was a British engineering company based in Millwall, London, England. They were involved in manufacture of marine engines, and some complete ships, as well as other diverse onshore engineering projects. An association with railway engines is usually attributed to G. and J. Rennie, which may suggest they used a second company to keep the books separate, and there was also George Rennie & Sons, which is associated with the development and patents of the steam disc engine. All three companies appear to have been in existence at the same time.

The Firefly was a class of broad gauge 2-2-2 steam locomotives used for passenger services on the Great Western Railway. The class was introduced into service between March 1840 and December 1842, and withdrawn between December 1863 and July 1879.