Firestorm is a Third Party Viewer developed by The Phoenix Firestorm Project and can be used to connect to both Second Life and OpenSim. It provides more functionality than Second Life Viewer. The most obvious feature that is has over Second Life Viewer is that it can connect to OpenSim as well as Second Life. It also has more options available to the user and in my opinion a better looking interface.
After using this viewer for a while it seems to run well in all the locations that I have visited in both Second Life and OpenSim without any problems that were the fault of the program and not the Internet connection.
My impression of Firestorm is that it is a viewer that is easy to use with a few options to customize it for the more advanced users so it is a good choice for any user of Second Life or OpenSim